Actor Allusion: In the two-part episode, "Brown Evil", Billy mistakes Hoss Delgado for a wussy video game character named Pat the Baker, both of which are voiced by Deiderich Bader. Similarly, in the actual video game of the series, when Hoss wins, he says "Bow to your sensei!"
Dean Toadblatt is voiced by the late John Vernon, who is remembered by many as the antagonistic Dean Wormer from Animal House.
Diedrich Bader as Hoss Delgado and Pat the Baker (a wussy video game character that Billy likes). Lampshaded in "Brown Evil", 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)!
The same goes for Boo Boo and Fred Flintstone when they had their cameos. They had their original VAs too.
In 'Here Thar be Dwarves' Michael Dorn (Worf and IM Weasel) shows up as King Beardbottom, and R. Lee Ermey shows up to try and block out the battle "for their younger viewers".
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.
One of Us: Fictional example. In the "Brown Evil" episode, whereas Billy prefers playing a cheesy cooking game named "Pat the Baker", Mandy prefers a much more violent Resident Evil parody featuring Hoss Delgado. At the end of the episode, Hoss states that he never leaves home without the former example and is seen cheerfully playing it alongside Billy.
Talking to Himself: There are a few examples in the series, the most notable being Billy and his father Harold (both voiced by Richard Steven Horvitz), and the few scenes that Grim and Sperg have together throughout the series (both of them are voiced by Greg Eagles).
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.