Hidbid.com is a completely free auction sniper and therefore better than other fee based sniping programs.
You can make 5 bids a week (not per month as someone said above) for free forever and you have the ability to edit your bids as many times as you want prior to the end of the auction. It is not an introductory free sniping offer like many of the fee based sniping programs have.
I have used them for a long time now. They will log into your ebay account and enter your maximum bid 5 seconds prior to the end of the auction.
Free for me
My last piece of advice is never ever bid in even increments. People think in 5 and 10s....Make sure you add random change to the end of any max bid you have. That way if someone who thinks in even increments bid $500, and you bid $500.21 ..you win!!! It's so true and has made the difference for me in the past.