'Of' as in 'I would of' makes me cringe every time I see it and want to bash the writer in the head with a blunt object.

Agreed. Also see: "of" used in place of "off".

The most retarded thing i can think of is when people verbally say abbreviations. I remember once someone actually said LOL verbally!

Just spotted on a.org: "Here here" and "Worse then".

*cringe*

How would you say them non-verbally?

How about "there" or "their" instead of "they're"?

Common mistakes that piss me off are... just about everything really... it'd be a hell of a long list to type out... so I thought I'd just lump it all together...

PS:One that really bugs me is when folk use 3 full stops at the end of everything... I mean WHY for gawds sake... get right on my diddies that does...