Diedrich Bader as Hoss Delgado and Pat the Baker (a wussy video game character that Billy likes). Lampshaded in the first episode featuring Hoss Delgado, when Mandy and Grim say, "it's Hoss Delgado!" while Billy yells, "It's Pat the Baker!".
Don't forget "Weird Al" Yankovic as the Squid Hat (and the announcer for the video game adaptation)!
Keep Circulating the Tapes: The first season was released on DVD but that has long since gone out of print, the same can be said for Big Boogey Adventure's DVD, several episodes were released either on Cartoon Network Holiday Compilations or as extras on other DVDs, thankfully the entire series (Sans Big Boogey Adventure, the Christmas special and Underfist) is available on iTunes and the first 39 episodes listed as seasons 1 and 2 are on Netflix.
What Could Have Been: The Underfist movie was meant as a possible pilot for a new spin-off series (made a bit more obvious due to the new animation style), but Cartoon Network turned it down. Some sources say that the creator, Maxwell Atoms, pitched the idea as a prank.