Badnarik. I'll have to check in with my Mom, a loooong time Republican who until the last time I spoke with her was actually going to vote for Kerry.
Nielson, don't mind Chairboy, he must've had too much coffee today.
Though there are only two biggies, you can vote for whomever you choose, whether that be Bush, Kerry, Badnarik, Nader, or Mickey Mouse. Whether the third party candidates (anyone but a dem or rep) gets on the ballot or not depends on state laws, which generally require a petition containing a certain number of valid signatures. If they're not on the ballot, you can simply write in your choice. Some look at voting for a third party as throwing away your vote...I look at it as one more vote toward making that party viable in the next election.