www.bidnapper.com
Been using it for years, and it's never let me down yet.
Why snipe? Because there are some silly people around who don't understand the principle of
Bid the maximum it's worth to you then forget about it until notified of the result. Some bid a bit at a time, so if you bid early they will outbid you. Others get so obsessed with 'winning' that they end up bidding some silly amount just to 'win'.
With a snipe, you are immune to both forms of idiot. Since I take the view that life is too short to sit hovering on an auction site to snipe manually, I pay 25p (for successful snipes only) to have a computer do it for me.