Aside from Penny Arcade and xkcd, I also read these three:
Multiplex
Basically, it's about the staff of a Cinema, and their escapades. It combines current movie references and good character development to make an overall good comic:
Misfile
After stuff goes wrong up in heaven, Ash Upton is transformed into a girl, and Emily McArthur loses the last 2 years of her life. While the angel who did this tries to fix it all, they have to go about their new lifes. Brilliant story and is rather boyish for being about a boy turning into a girl. Heck, it convinced me.
AmazingSuperPowers
It's dark humour, and it's hilarious. Probably my favourite comic of the lot, and is highly recommended. See for yourself: