I've been using
http://www.esnipe.com and it ROCKS !!! I've won many auctions using it. It's great because I'm not much for remembering to check an auctions, so when there's something I want, I just head over to esnipe and put in my bid for it to do the bidding for me at the last second
They have a free trial and once it's over, you just buy credits to use which cost like 1% of the auction total, so if you win something for $50, it costs you 50 cents, that's definately worth it.
Esnipe even has a nice feature which is beautiful. Say for instance you want an item and there's 4 auctions running. Well, you can have Esnipe bid on each item before it ends, but what it does is wait for you to win one, then it cancels the other bids.
You DEFINATELY wanna snipe the auction at the end. You NEVER EVER EVER EVER wanna bid on it before then because that's just gonna bring the price up higher. Say you outbid someone, well they're either gonna outbid you or watch the auction closely at the end to jump back in.