Cash Cow Franchise: Within its first 10 years, Spongebob made over 8 BILLION DOLLARS in revenue. No doubt that it's still making more.
A recent study has shown that it currently accounts for more than 60% of the station's revenue.
Cross-Dressing Voices: Patrick and Mermaid Man are voiced by Ikuko Tani (an old lady) in Japanese; Plankton, however, is voiced by Chie Matsuraa, a young woman who doesn't even try to match Plankton's English voice.
The Danza: In "Mermaid Man Begins", the real names of Mermaid Man and Barnacle Boy are revealed to be Ernie and Tim. Mermaid Man and Barnacle Boy are voiced by Ernest Borgnine and Tim Conway, respectively.
Enforced Method Acting: Tom Kenny actually was very sick when recording Spongie's sniffly, sneezy dialogue in "Suds."
Executive Meddling: It has saved the show from two series finales. The movie was intended to be a Grand Finale for the series, but the possibility was later pushed to the "Spongebob's Last Stand" special. The show was continued since it was very close to dethroning Rugrats as the longest running Nicktoon. It eventually did bypass the series when it aired episode 173.
In the "SpongeBob B.C." special, Bill Fagerbakke portrays (in live action) Cavey the Caveman, and when he speaks, one can easily tell that he's Patrick Star's voice actor. Tom Kenny averts this trope (at least in this context), as his portrayal of Patchy the Pirate sounds nothing like SpongeBob's voice.
Singer Pernilla Wahlgren does all the speaking roles of the main female characters (Sandy, Pearl, Mrs. Puff and Karen) as well as many, and I mean many additional voices. The only female character we know for sure who isn't voiced by her is Princess Mindy in The Movie.
Recycled Script: A few old episodes of the show seem to have recycled plots from other Nicktoons. "Toy Store of Doom", for example, has essentially the same plot as the Rugrats episode "Toy Palace" (they get locked in a closed toy store and are afraid the toys will attack them), while "Banned in Bikini Bottom" (Krabby Patties are outlawed and Mr. Krabs starts selling them at SpongeBob's house secretly) is similar to the CatDog episode "Just Say CatDog Sent You".
"Picture Day" has a recycled script from an episode of a Disney show: the Recess episode "One Stayed Clean". "Big Pink Loser" is also extremely similar to "Copycat Kid".
"Sailor Mouth" is very similar to "Bleeped", but airs much more frequently.
"Squid Defense" is very similar to the Hey Arnold! episode "Mugged"
Screwed by the Network: On their Nicktoons channel, as it only airs when most people are asleep. You were probably confused when you saw this trope on this page if you don't get Nicktoons. (That's probably because they don't need to air it on Nicktoons.)
In terms of the series itself Word of God also states that in spite of the series continuing to this day; the original team members and the Hillenburg himself say that the first theatrical movie is the end of Spongebob's story.
As the second movie is starting to be filmed in Savannah, it's anybody's guess as to where the story stands now.