1/0 lifted its first character, "Barnacle" Jones, from the comic Absolute Tripe, which introduced him as "the first man in history to seal a fart in a mason jar". The jar came along with Jones, though 1/0 never shows it in-panel. Later in the strip, characters begin using "Jar Breeze" as profanity.
Junior attempted to use the author's name (Tailsteak) as a curse word for a period of time because he reasoned that the author can't bleep out his own name.
Achewood features Philippe, a five year old otter, who obviously can't be allowed to swear like everyone else in the comic can (and does), on account of being five forever. For example, when Philippe comes across straight man Teodor watching gay porn and is informed that it's really Superman helping another man out of the shower: "Superman wouldn't wear a police hat in the shower! Applesauce!"
This usage of "applesauce" is actually vintage 1920s slang. It's used in Who Framed Roger Rabbit, among other places.
In Adventurers!, one strip plays with the censorship on RPG curse words, using various ways of covering up the words themselves, eventually using "Spoony", a play on a famous scene from Final Fantasy IV. After that, the word "spoony" became a running joke in the strip, with characters spouting it all the time in place of curse words. Other utensil-related words were used, such as "spoon" on its own, "Fork you", or even "utensil", but "spoony" was the most widely used.
Badly Drawn Webcomic takes this to the logical extreme in this strip.
College Roomies from Hell!!!!!! has used, among others, "xlempaphroggin'" and "spiroratstar." Real ones are used sometimes, though; they're just scribbled out.
Astute readers will notice that "spiroratstar" is actually a written-out rendering of an instance of Symbol Swearing that appeared in an earlier strip.
In Erfworld, Parson finds that any cussword he attempts to utter (and he attempts to utter several immediately after being summoned) comes out as "boop". Apparently, the "game mechanics" that govern Erfworld include a profanity filter.
Parson thus likes it when he finds a curse not covered by the filter, and in the meantime has begun to use some unusual language of his own.
For a while, Everything Jake had "Quck" which even the other characters were unsure of.
In the Jenniverse, starting with Unicorn Jelly, odd runes were used where the offending words were used, but a more common and understandable version was the multi-purpose 'Farg'. Since the language they spoke was supposed to be a compromise of mostly english and some other languages, this makes sense.
Specifically: Hebrew, Greek, and Japanese. But those were the lesser-used (in-universe) Alpabe runes, whereas the glyphs that replaced most other offending words were Talcryl script.
An example of a Talcryl symbol with a pretty obvious meaning.
The Cyantian Chronicles has "Squid", which is used as a catch-all curse word for cyantians. Normal curse words get censored. The origin of "Squid" as a swear word comes from the period where the whole of Cyantia was enslaved by the Moulin Phedra, who happened to look a lot like Squid.
"Squid on a Stick" is my favorite occurrence of this.
"Dratsad", which is used instead of "Bastard!".
David Gonterman censors his own work relentlessly, so he's come up with quite a few of these. Phrack (guess what it's a substitute for) is a particularly common one, but there's also the infamous "Clinton Jobs".
Dan and Mab's Furry Adventures uses the word "frig" a lot (especially Dan). Which is also a real-world euphemism. Mab, on the other hand, yells "Friggernaffy!"
In one strip of El Goonish Shive Grace and Sarah are talking about Sarah and Elliot "getting a room". The title of the strip "clarifies" that they are referring to "playing boardgames". "Playing boardgames" has now become a euphemism used by the EGS fandom. Considering how EGS has also mentioned at various points "Strip Scrabble", naked games of Twister, and the "Best. Card.Ever!", this euphemism is not unjustified.
Carrie from Everyday Heroes tries not to swear since she's a religious girl. However, she is occasionally known to exclaim, "Saint Francis University!" St. F. U.* Cardinal in Finders Keepers uses "Compass and Cross!" as an exclamation of shock and frustration in lieu of swearing. Cailyn, on the other hand, drops regular old-fashioned F-bombs when she's stressed.
Leslie from Friendly Hostility uses *swear* as the author does not know a word strong enough for him to say.
We get the amusing "poofers" in the webcomic Hamsta Powah. The idea behind the word is that, due to the fall of humanity and small creatures artificially evolving as the new intelligent race (so far, hamster, mice, rat, squirrel, and rabbit furries have been seen), new swear words have popped up. "Poofers" translates to "crap" or "shit" when used in conjunction with disapointment at something happening. A notable example is when Sam said "Oh, poofers" before being hit with the tornado from Hiate's Sky's Fury summon.
Author and artist Sam Boyd says the word is really just because, quote "hamster profanity is fun".
Possibly Averted/Subverted/Funny Aneurysm/Did Not Do the Research in that 'poof' is a British slur against a homosexual man; particularly one that exhibits effeminate mannerisms. Calling someone a 'poofer' (particularly in the 1960's, as in modern parlance it becomes 'poofter') is a distinctly nasty thing to do, but not particularly unusual.
In a Lackadaisy side comic, the author points out how some fans wonder if Zib "is a viper", having a "Texas tea party", doing the "golden strut", "kicking the gong around", "copping a deuceways", "courting the white lady", and having a "whizbang good time". They're all euphemisms for drug use.
In Loserz strip here, "butt-noses". Almost adorable, isn't it?
Piro and Largo from MegaTokyo say "Fsck!", which is a UNIX command that scans the hard drives for errors and fixes them. Since its usage is common in hacker culture, this insult makes sense, since Megatokyo features a lot of influence from the hacker culture.
The legend behind the fsck command is that it was originally more colorfully named, being what one types when the disk is broken, but that the name had to be changed to satisfy lawyers in an early release; filesystem consistency check is thus a backronym. Or, as the Unix Haters Handbook puts better, "think of a good English word to describe what you want to do, then think of an obscure near- or partial-synonym, throw away all the vowels, arbitrarily shorten what's left, and then, finally, as a sop to the literate programmer, maybe reinsert one of the missing vowels."
Various couples in Namir Deiter have expressed fondness for "Cookies and Pudding".
Neon Ice Cream Headache has several odd terms for drug use, such as "Raking the leaves off the neighbor's lawn", "Sipping ice tea from a Chinese slipper", "Waking up with Winston Churchill", and "Crushing a tiny porcelain poodle with your bare hands".
While the dwarves of Oglaf's "fukken" may seem to be only a slightly unique pronunciation of an obvious word, their tendency to use it as their only swear and interject it at seemingly random points leans more to this trope.
The note for one Schlock Mercenary strip: For our younger readers, "alone time with my wife" is a euphemism for "playing scrabble." Haban doesn't want Petey to know he got the word "stimulation" on a triple-word score.
Later, the readers are given "Charlie Foxtrot" and interesting derivatives, such as "Charlie danced the Foxtrot." This one actually makes perfect sense: Charlie Foxtrot is the Military Alphabet phonetic version of CF, which is short for clusterfuck.
Later on Kathryn Flinders uses "ficus foxtrot" and, on one particularly harrowing occasion, "creeping Charlie ficus foxtrot".
The phrases "crap on a crutch" and "hellfire and blamnation" are occasionally used as well. According to writer Howard Tayler, these are profanyms (or, more specifically, blasphenyms): words that sound naughty, but aren't.
"Hullnuts" also turns up frequently, usually in a context where "hull" could be left off entirely and it wouldn't make any difference.
This is the sole purpose of one character from Sexy Losers who is only called Touro's friend (swearing). The creator acknowledged that his speech kind of makes sense but really doesn't it's just meant to sound insulting and use a lot of swearing. He's given us such gems as "Yeah spooge mouth. You fuck cows in retrospect" and "Yeah fuckwit you shit for sale."
Nick Zerhakker has censoring software put in him, which substitutes similar but clean words much like broadcast versions of movies do. Its choices get increasingly baffling.
Then there's the time Unity is in New Orleans and calls Tip and Sweetheart.
Unity: Me, I'm stuck here punchin' the gator. Sweetheart: What... does that even mean? Unity: I totes cannot make this clearer. I am punchin' this gator. Sweetheart:Oh. Unity: In the face. Sweetheart:[to Tip] It's so hard to tell when Unity is euphemizing. Tip: Like when she throttled that bishop?
Sluggy Freelance, within the much-maligned "Oceans Unmoving" plotline, has scientist Kada frequently uses the interjection "splat" and euphemism for generic curses. After the Carib "Honest" Stu gets shot and is dying, Kada swear revenge, referring to the attackers as "splats" and Stu interrupts her to tell her that "splat" is actually a derogatory term for Caribs themselves, derived from the sound they made when being run over. Cue Kada's horrified backpedaling followed a beat panel, then Stu saying "Now I know why you guys lie all the time. It's funny!" before dying.
She also uses "freg" to stand in for the f-word—generally at least once per comic she appears in. Yeah, she's pretty foulmouthed.
Don't forget to ask Gwynn about her Monkeys. Especially if they have escaped and you need your friends to help you look for them.
Suicide for Hire got one in a Continuity Nod: an early comic depicted a fully-clothed Talking Animal turtle. Clothes have to be a weird shape to fit over a shell, and this got a Lampshade Hanging with an author's note reading "Don't ask me how turtle shirts work. They just do." In a more recent strip, Arcturus sees something shocking and yells "What the turtleshirt is this?!"
Terinu makes frequent use of "Frell" (borrowed from Farscape) and "Fragg", both standing in for the usual F-word. Word of God reports that "Fragg" came from her husband's attempt to not swear in front of their children, instead substituting Fraggle Rock!
Thinkin Lincoln has the occasional use of lines such as "What the crumpet" and "Son of a birch."
The Wotch. Anne Onymous often says "fish sticks!" instead of a more objectionable expletive.
The Walkyverse gives us "Cheese" and "Cheesus" in place of God in most contexts. "The Cheese" is a nickname for a godlike entity in the 'verse, which half-explains it, but several characters with no idea who the Cheese is use it from time to time.