Alternative Voting Systems

In an interesting discussion at a family BBQ last night, a few of us were discussing alternative methods for tabulating votes, as opposed to the "just vote for who you want" system we have now. My wife's brother-in-law suggested this:

You can either cast a vote "for" somebody that adds to their votes, or "against" somebody that subtracts from it. Hypothetically, you could have the McCain and Obama supporters cancel each other out by voting against the opposing candidate, and wind up with a third party winner who had fewer "for" votes than the other two.

I like that one a lot, it would be very entertaining. My suggestion was conditional votes:

You can cast a "preferred" and "conditional" vote. This would require two rounds of counting. You cast a vote that lists your preferred candidate for the office, then a second choice conditional on your preferred candidate not being in the top two for most preferred votes. So suppose you vote preferred for Libertarian Bob Barr, with the conditional being Obama. If after counting the preferred votes, if Bob Barr is not in the top two of the preferred candidates and Obama is, then your vote would be given to Obama.

I like that idea because it would make the system more competitive and you could overcome the objection to voting for third parties ("a vote for Nader is a vote for Bush", etc. etc.). However, I'm not entirely convinced that increasing political competition is actually good.

Other Alternatives? Thoughts?