Pretty much anything from Despair.com is hilarious. Personally, I've always been a big fan of the Demotivator calendars. I keep thinking I'm going to make my own customized one, but I'm too lazy to put in the amount of time needed to make it truly awesome. Thus, I'm probably also too lazy and uncreative to exploit the Parody Motivator Generator to the fullest. But if you've got a clever photo/caption combo, knock yourself out.
Meanwhile: I'm on the fence about getting my demotivation in t-shirt form. This one is pretty funny, though.
And now there's one that directs the slam back at the wearer.