I'm about 98% sure that's Stephen Root.

Nobody voices cosmic crazy like Stephen Root, he is a national treasure.

Oddly enough, the Internet doesn't seem to know which voice actor played him.

Ah, yes, it's Stephen Root. http://frederatorblogs.com/adventure_time/2011/01/03/the-other-tar ts-animatic/

Attack ships on fire off the shoulder of Orion.

The first season is still airing and this episode is from two weeks ago. Hopefully they'll start releasing DVDs at some point, though CN admittedly doesn't have a great track record in that department. Flapjack is still lacking a decent DVD set even though the show is over and they aren't bothering to re-run it.

Thurop Van Orman mentioned the marketing/DVD department at Cartoon Network is immensely shitty. Which makes sense, given that it took them 15 years to release a Powerpuff Girls collection on DVD.

At least Thurop is working on Adventure Time now, and that's already been renewed for a third season.