VeriSign are huge. They are more than just a signing authority, they run and control a fairly major chunk of the Internet backbone (including several root nameservers and the .com and .net top level domains) with ambitions to control far more. Were recently involved in a bit of controversy by redirecting non-active or non-existent domain requests to their own sites.
Thawte is the other major one... who are also owned by VeriSign
Then there's
GeoTrust... yep you've guessed it, also now owned by VeriSign
They're all the ones of any repute. Note that the likes of Microsoft back them so guaranteed to have the root certificates in your browser and no problem authenticating anything signed by these guys.