Phil LaMarr also falls under this. Before he did voice acting in shows like Samurai Jack (as the title character) and Futurama (as Jamaican accountant Hermes Conrad), he was a cast member on MADtv (though before MADtv, he had a small, but memorable role in Pulp Fiction [which explains why in the show's parody called "Gump Fiction," LaMarr screamed, "Not again!")
Indeed, several of MADtv's actors have found successful careers in voice acting (with the exception of Taran Killamnote who is currently on Saturday Night Live, Jeff Richardsnote who was on SNL, then left for different projects, Will Sassonote whose recent credit was as Curly Howard on the movie adaptation of The Three Stooges, Matt Braunger, and Bobby Leenote both of whom have mostly been doing stand-up since MADtv ended). Unfortunately, many of the later performers (like Crista Flanagan and Nicole Parker) have found themselves stuck in the Seltzer and Friedberg movies or on failed and/or short-lived Comedy Central shows (with Key & Peele hopefully being the exception to the rule, as it was praised for being the best sketch show on the network since Chappelle's Show).
Mo Collins is currently Recurring Character Joan Callamezzo on Parks and Recreation. Joan has a somewhat antagonistic relationship with the main character Leslie Knope, played by Amy Poehler. So essentially, a MADtv alum is playing the rival of an SNL alum (how apt).
Ike Barinholtz (who was on the show from season eight to season 12; known for his impressions of Dane Cook, Nick Nolte, and Kevin Federline, and often paired up with Josh Meyers in sketches) was on this show before he was on The Mindy Project. He left after season 12 due to disputes with the executive producers and overall boredom with the show.
Keep Circulating the Tapes: The complete first season and a "Best of" Clip Show featuring the best sketches from seasons eight, nine, and ten are the only officially-released DVDs of MADtv. Unless you videotaped all the episodes from 1995 to 2009 or taped reruns from Comedy Central (or Comedy Network), then you're S.O.Lnote shit outta luck in finding episodes (especially the ones made during the show's final years, as neither Comedy Central nor Comedy Network reran those. Comedy Central aired up until season 11 while Comedy Network aired up until season 12).
Shout! Factory (the same people who put Rocko's Modern Life and other shows on DVD) announced last year that they just acquired the rights to MADtv and will release the second season on DVD in March of 2013. However, since most of the earlier episodes had musical performances from popular bands of the time, the episodes on DVD may be cut due to licensing issues.
Sibling Rivalry: Josh Meyers (a cast member on MADtv for the show's eighth and ninth seasons) is the younger brother of Weekend Update anchor Seth Meyers in Real Life, making Seth Meyers the only cast member to have a sibling on a rival sketch show. Made evident when Seth Meyers appeared on Weekend Update to talk to his dad and tells him to turn off MADtv.
What Could Have Been - Seth MacFarlane (the man behind Family Guy, American Dad!, and The Cleveland Show) was originally contracted to do cartoon shorts for this show, but backed out in favor of creating his own TV series for FOX. Had MacFarlane taken the offer, he would have created Family Guy for MADtv before spinning it off into its own show, much like Matt Groening did with The Simpsons on The Tracy Ullman Show.
Keegan-Michael Key and Jordan Peele were originally hired with the intention of FOX picking only one of them to be in the next season's cast, but the two had such great chemistry both separately and together that FOX decided to pick them both.
Stephnie Weir (the MADtv cast member known for recurring character Dotnote a mentally unstable girl who's parents abuse her because she's not smart like her sister and her impression of Anna Nicole Smith) originally auditioned to be a Saturday Night Live cast member. She was passed up on SNL in favor of Rachel Dratch.
Jordan Peele also auditioned to be on Saturday Night Live during the hiatus brought on by the 2007-2008 Writers' Guild strike when Lorne Michaels used the time off to find a cast member who could play Barack Obama. Peele and another black male cast member (Donald Glover) were passed up in favor of Fred Armisen (to much controversy) until September 2012, when Jay Pharoah was chosen. The joke's on SNL as Jordan Peele is now Barack Obama on Comedy Central's Key And Peele.