According to Muppet Wiki, Boober, Skeksis and Oscar were all on a recent episode of South Park about an imaginary world being under terrorist attack
Awesome I was going to mention that one. I remember seeing Boober, sadly killed,
. The Skeksis was part of the "evil imaginary characters" gang and I didn't see Oscar though, I'll have to keep an eye out for him next time I wacth thoes episodes. On a personal note I really loved that three part episode, I already can't wait for season 12 to be on DVD just for thoes three episodes alone.
Also I don't know if anyone mention this already, but on an episode of "Drawn Together" entitled "Drawn Together Babies" which was a parody of "Muppet Babies," although I'm not a huge "Drawn Together" fan (I think it has it's moments but for the most part, I believe that show tries so hard on the "Shock Value" that sometimes it pasts being funny. Just my say), but they did a pretty close verison of the Muppet Babies opening. I really enjoyed it.