Common spelling mistakes.
It's a lot, not alot.
It's all right, not alright.
It's definitely, not definately.
It's separate, not seperate.
It's weird, not wierd.
It's lose (to express you have lost something), not loose.
It's its (to express possession), not it's (which means it is).
Don't worry, we're all guilty of using one of these at one time or another.