Frogger just beat me to it... I read through all 20 pages thinking why hasn't anyone mentioned affect and effect !
For me, the two main offences I see are: their/they're/there and your/you're. As they say, punctuation is "the difference between knowing your cough, and knowing you're cough".
May I be indulged to add that irony (and what is ironic) is probably not what you have been led to believe. Most of the things people say are ironic aren't, the're unfortunate. Almost everything in Alanis Morrisett's song is not actually ironic. The conclusion of Romeo and Juliet is tragically ironic because he kills himself believing she is dead unable to live without her, when ironically, she feigns death to be with him (actually, but not ironically, Shakespeare understood how to use English effectively).
OzEire
ps. this thread is for English pedantry, all that is left for me to say is "pog mo thoin"