I usually pop anime in right before I crash out for the night so this is exactly the kinda stuff I'm watching right before I doze off. I definitely have some of the suggestions listed here (like Doki Doki School Hours, Ichigo Marshmaro, etc.) but my all time fav is Ping Pong Club. This is a series people either love or hate but I absolutely love it. It's kinda a mix of Revenge of the Nerds, Beavis and Butthead, and South Park all in one Anime.
I would also recommend City Hunter, Genshiken, Midori Days, Lupin the 3rd, Tentai Senshi Sun Red, and Ninja Nonsense. If you're okay w/ more fan service than I would recommend Girls High and Golden Boy. All titles have a nice dose of comedy and always make me laugh no matter how many times I've watched them.