Keith,
I think
Cream was probably the first Supergroup, combining Clapton from the highly succesful
Bluesbreakers 1965 album with the Rhythm Section of the
Graham Bond Organization, every member being already a big name on the UK scene.
THE PAST
Crosby, Stills, Nash (& Young) - yeah, one of the earliest Supergroups!
ELP are certainly another favourite, and the highly underrated
U.K. of course!
I wonder what would have happened if
Baby Face had ever gotten beyond jamming (-> Phil Lynott, Ritchie Blackmore and Ian Paice!)
There are actually some
The Firm tracks I do enjoy. They had their moments, but the timing wasn't probably right.
Firm bass player Tony Franklin was in another kind of a Supergroup though -
Blue Murder, with John Sykes and Carmine Appice

Fanastic first album.
While I always thought that
Asia, especially the first album were great
GTR were very disappointing
THE PRESENT
Supergroups have practically been non-existant for years. Yet with
Chickenfoot,
Them Crooked Vultures and more possible alliances on the scene, it seems like they are becoming en vogue again.
Terry Bozzio, Allan Holdsworth, Tony Levin and Pat Mastelotto are currently touring. Another supergroup, huh!
And
Steve Hackett and
Chris Squire are still preparing a joint album and tour with
Simon Phillips
And the mighty
Transatlantic have just released their first album after a 10 year hiatus - that's Mike Portnoy (Dream Theater), Neil Morse (ex- Spock's Beard), Pete Trewawas (Marillion) and Roine Stolt (Flower Kings) FYI.
Certainly a supergroup.
Quite some namedropping here. Looking forward to reading more thoughts and suggestions!
Cheers,
Christian