I'd second TheDustyZebra's Retief (and Laumer's written several other humorous pieces, many of which are collected in The Lighter Side, though I haven't read it yet, so can't recommend it yet). Also Tim_Eagon's Robots Have No Tails and Victoria Silverwolf's The High Crusade, though the latter is incidentally funny, if I recall correctly, rather than outright comedy.
To add some partly edge recommendations:
Frederik Pohl and C.M. Kornbluth's The Space Merchants is satirical and bitter rather than "humorous" but I recall it being very witty and funny.
Seems like James H. Schmitz's The Witches of Karres worked as a straight, somewhat light adventure novel but did have humor woven in - I know you weren't asking for incidental humor but this may be close enough. Good book, anyway.
Much early Rudy Rucker, at least, is hilarious. It's written for a variety of reasons, not primarily humorous but it has a gonzo style throughout and an often cracked perspective that makes for humor as well - especially Master of Space and Time which, for some reason, I think of as Kuttneresque in ways.
Jonathan Lethem's Gun with Occasional Music is a hardboiled detective novel and a science fiction novel and... it's funny. In a dark way. I mean, it's got gun-toting kangaroos.