There are few sins worse than voting for a person you know is the wrong person.
I say that you should vote for a third, fourth or hundreth party (yes, there is a hundreth party in nearly every presidential election). They are there, and the only way they will gain power is by your votes. Granted, they aren't going to win this time around, but this is a long slow battle. One day enough of us will be (with your help) voting for other parties that an election could actually mean something.
You may not change much, but you will be doing more than nothng, and you can finally vote for someone whose ideas you like.
On the other hand, if there is no one running that you are impressed with, remember that some of us vote for poeple who we truly believe in. If you do not vote, you will make our vote count for more -- most likely a Good Thing. Don't vote just for the sake of voting!
See also:The message that not voting sends, blank ballot project.