I use (you need an account to see links)
Works well. Click the item in the auction, auto-bids, goes back to the item, refreshes the page until you aren't the top bid and auto-bids again. Works quick, have sniped a number of things using it. Just make sure you don't accidentally click an item because it places the bid very, very quickly. I only enable it when I actually want to snipe something just in case I forget about it and bid on something I don't want.
Edit: Make sure you adjust the "settings to edit" portion of it. Personally I have it set to,
var maxBid = 100000000; //max you are willing to bid
var minRef = 1000; //min refresh time (ms)
var maxRef = 1000; //max refresh time (ms)