Can't leave Roy from the Order of the Stick off this list. As the Only Sane Man in a party with a sociopathic halfling, a Spoony Bard, a greedy rogue (is there any other kind?), a dwarf who's afraid of trees, and a power-obsessed elf, Roy has played the Straight Man to each at least once and countless times for most, especially Belkar and Elan.
While Roy was dead and the gang was split, Haley became a Straight Man to Belkar and Celia, while Hinjo became the Straight Man for the others. Haley still often plays the Straight Man to Elan.
Cherry is the Straight Man to Hero in the webcomic RPG World. She is, in fact, prophesied to be the Straight Man for the entire world, as she is the only one who thinks there's something strange about a world where all battles are turn-based and groups look like their most important member when viewed from afar, also making her The Chosen One.
Shortpacked! has theorised that Batman can make anything funny, because he is the ultimate Straight Man. When you consider that his arch-nemesis is the Joker, we may have the oddest Odd Couple in history...
Ethan in the main storyline originally played Straight Man to the rest of the cast (even though he's not straight), but as time went on, his quirks became more pronounced, and now he's just as wacky as everyone else (albeit with more frequent moments of clarity). Now his role is filled by the newcomer, Jacob (who actually is straight).
In ShiftyLook's comic based on Dragon Spirit. The main character Amul is the Comic Foil while Nida is the Straight Woman.
Sam in Sam and Fuzzy spent most of the comic's early run being Fuzzy's and Fridge's straight man (though practically everyone is the straight man compared to Fuzzy). Although the comic has since turned a lot more story- than gag-driven, Sam is still usually the one who sets up the panel jokes for more eccentric and/or psychopathic characters, like Sin and the Ninja Mafia.