Wish I had mod points today for this one..... preferential voting / STV, whatever you call it would be awesome. It would allow 3rd and 4th parties to slip in and flip the duopoly we presently endure in the USA.
Something I do get a kick out of is when people say that some random 3rd party under cut their candidate du jour. They assume that 100% of the third party voters would have voted for their candidate. This isn't true. Take the libertarian party... they want smaller government (attractive to republican voters) and legalization of many drugs (attractive to democrat voters). So I don't really think the 3rd party "spoiler effect" has much validity. The bigger problem is when the top two parties morph so much that they loose their base voters. As an example black people generally vote democrat, but the democrat party lately has started courting gay demographics. This has a negative affect on the black voters who then feel alienated from the party they once supported they stay home or vote 3rd party.