Definitely, otherwise known as "Definately, defenitely, definetely, difinitely," or my personal favorite, "[i]defiantly.[/i]"
[spoiler]There's not even an 'A' in the word. How can you get that shit wrong?[/spoiler]
OTHERS:
There and Their - Mix-up
Hear and Here - Mix-up
Your and You're - Mix-up
Tomorrow - Tommorow, Tomarrow, Tomorroh (seriously saw this once.)
-
Can not/Cannot Hes/His Know/No There/They're/Their Your/You're Wensday/Wednesday Knew/new And of course comma splice. Comma splice everywhere.