What is the best eBay sniping tool?

I don't know that there is a "best" eBay sniper.

A sniping service or software must be very reliable, and secure (uses SSL, etc.). After that, it comes down to taste.

Some users may like the layout or colors of a sniping site better than others.

Some users want a very simple interface with basic functions, some users will want every bell and whistle that you can think of that could go along with a sniping site.

To add to the site that was already listed here -

Hidbid.com

Bidball.com

Or about five other sites that seem to be good services.

There are a few good free services, I haven't used any of the pay services since the free ones have been fine for me. I'd recommend:

gixen.com

myibay.com

GavelThief.com, you could try

http://www.snippys.net (for eBay and Delcampeauctions websites)

Both are free for unlimited snipes, Gixen has a very cheap premium service that looks interesting (gives you desktop software, mirrored sniping server, and autosnipe browser extensions, among other things). Myibay seems to make their money from donations & selling their desktop software (the web-based version is entirely free). As for the free versions, Myibay has more features while Gixen seems easier to use.