Most of these are ones I've thought of myself, but I have researched a few choice ones. Corrections are bold, in the brackets.
Should of / would of / could of. (Should have / would have / could have).
All of the sudden. (All of a sudden).
Used to. (Find a synonym for this. Though it's technically correct, this phrase doesn't read well in text).
On accident. (By accident).
For all intensive purposes. (For all intents and purposes).
Good vs. well. (I am doing well = I am healthy and content. I am doing good = I am Superman and I am saving kittens from trees).
It's a doggy-dog world. (It's a dog-eat-dog world. Are you kidding?).
Hunger pains. (Hunger pangs).
The spitting image. (The spit and image).
So, there you go. Some were obvious, some were a bit more obscure. If you have any more, feel free to add your own page!