Hi T.,
I can understand what you are going through, our son went through the same thing (and fortunately it is a phase), I tried to follow Dr.s Sears advice on "BITING AND HITTING: 16 WAYS TO STOP IT" http://www.askdrsears.com/html/6/t063900.asp
They basically say at this age children don't have the verbal language to communicate their needs.
Try to figure out what triggers the hitting, (tried, hungry, feel out of control).
Teach them to "hi-five" instead and firmly, but calmly say, "no hit!".
If you took something away, you can verbalize the child's feelings (I understand you wanted X and now you are angry).
If the child hit you that day, mention it to Daddy (in front of the child) and then let Daddy sit the child down too and say that it will not be tolerated: "I will not allow you to hit the woman I love".
If you are with other children in a play group, keep a close eye on him and don't hesitate to tell the other moms (they have been in the same or similar situation and they can understand what you are going through). But if you don't mention it to the parents then when your child hit's their child they are angry that their child has been hurt. Then you could get blamed for the child's misbehavior ("bad parents of a bad kid"), and the adult friendship cools. I've learned that through all of the stages our children go through we really need good friends to bounce things and ideas off of and we need their support to stay sane. haha
Good Luck!
C.