Snapnames is the most acclaimed service around with the longest track record. ($69, or as low as $39 at some resellers)
If you can get a backorder in at dropwizard.com, do so! ($120)
Namewinner is also a big fish at current and have caught a ton of great names in the past six months. I'm using them less and less because the bidding is just getting out of hand.
GoDaddy has recently began a domain backordering system and has beat Snapnames on several occasions already. I've just started using them so I don't have my own results to report yet. It looks promising though.