Hi whitelion
Snapnames.com and NameWinner.com are the two more reliable services.
NameWinner is a bidding service - whoever has the highest bid gets the name (assuming they grab it). Some of the bids can get ridiculously high.
SnapNames is a first-come-first-served system, with a fixed fee ($69).
Snaps.com, DomainSnaps.com and NameAlerts.com are actually just resellers of SnapNames. Purchasing a SnapBack at any of those services is the same as purchasing one at SnapNames.com. The only difference is that you cannot exchange the SnapBack to another domain more than 3 times (but they're about $30 cheaper). At SnapNames you can exchange an infinite number of times (within a one year period), until you catch a domain.
There are other services around:
DropWizard.com ( used to be great, but have gone very quiet :sad: )
NameFox.com ( Very uncommunicative )
eNom Club Drop ( Moderately Successful )
GoDaddy Back-Order ( Stay Away )
TheFirstName.com ( Were going really well about 1 month ago )
Hope that makes sense