Biggest one I've seen on forums and everywhere else that bothers me the most is:

"I could care less" when people mean "I couldn't care less."

Not only is the phrase people use incorrect (that bothers me a little bit) but it is actually the EXACT OPPOSITE of what they want to say (bothers me the most). "I could care less" means you care so much about something that you could, if forced, care less about it. People don't even bother to stop to think about what they say or type.

Yes! I hate that one! Then there's that usual "your/you're" thing and "their/they're". Seriously people, the first one means possessing something, the latter means being something. Your problem is that you're too careless or ignorant.

I can't even remember all of these... There's tons of easy mistakes people do just because everyone else does so it suddenly becomes acceptable!

it's a constant that will never change.

X "sales" Y units, instead of X "sells" Y units.

the interenz is doomed because people can't spell or write the right word when it is needed.

the easiest way to not be irritated by something is to either ignore it or enjoy it for what it is. it really doesn't matter how much people complain, these things will always be there as not everybody has English as their first language

it's my third btw so excuse me if I make mistakes now and then

I came here to say this. I see my work is already done.

I guess I'll mention other confused words like

Site/sight - It was a site to see instead of It was a sight to see.

Quite/Quiet - I'm pretty sure this one is a typo because these words don't sound the same at all but I've seen sentences like "It was quiet impressive" quite often.

Nothing is worse than would of/should of/could of. How the fuck can you not know what the word "of" means?!

Also, when the phrase "for all intents and purposes" is mangled into "for all intensive purposes".

My English is very poor, but even I don't make those grammar mistakes :p

I have never agreed more with anyone than I do right now.

I must admit, round here people seem to get it the other way around.  At least what I've seen.

"XYZ makes 123 sells"