Hi R. -
While I didn't have a "hitter" when my son was younger (he is 9 now) he had major issues with angry outbursts where he would tear things up, throw things, slam doors, call you angry things, scream, etc. Anyway, it was miserable. I tried everything! Or, so I thought! What I didn't realize was part of the issue (or the majority of the issue) was my husband and I's REACTION! Reaction is huge in factoring in to how our children behave. Kids do what pays! There is something he is getting from this behavior. My guess is that it is attention! And to a child attention is attention! You need to come up with a catch phrase for hitting and follow through with it each time! The follow through is very important (think consistency). Say to him, "(Insert name here), I am sorry you are mad or upset but hitting hurts people, and hitting is NOT ok. When you are calm and not hitting we will talk about it." Or say something to that affect and put him in a safe area away from you/ his toys/etc. and WALK AWAY FROM HIM. DO NOT respond any more in any way.
Mistakes I made with my son were to try to "talk" him out of negative behavior or yell at him/get angry right back. Remember, you want to model what you want from him in return...so if you are super mad and yelling that is most likely what you will get from him. Teach him words to express when he is frustrated - "Mommy I am feeling very angry." And watch for what tends to set him off. Then you can hopefully address that before it gets to the hitting point. Two year olds are impulsive and he is acting on impulse. This is pretty normal. You just have to reinforce what you want from him. Catch the moments where he is playing appropriately and praise him..."wow, you sure are playing nice, I really like to see you treating your (toys, friend, mommy,etc.) so nice."
The main thing he needs to learn is that hitting is not ok and he will not be allowed to do it. When he does if there swift action/consequence that follows he will get the hint and will get better about expressing himself in a more healthy way when he is upset.
Lastly, I highly recommend you read Love & Logic (or if no time for reading get the audio versions from your library). A lot of teachers, daycares, and parents swear by it. I used it and it made a wave of difference in my son when virtually nothing else was working! And remember, "this too shall pass." Hang in there!
Roxanna