If you search "Cyanide and Happiness" grammar in Google, there is a mind boggling array of grammar humour. It's so funny it makes me snort. It was hard to pick just one!
(A side note: Phill says he's told me about this site before, but as my memory is shocking, I get to enjoy the discovery each time like it's brand new!)
**Disclaimer: Please note, that many of the cartoons on this website are quite offensive... I only looked at the grammar ones today, because I'm a big nerd.