Nichols & May
JTS Brown reunion
Heather & Miles
Convoy (before Heather & Miles' UCB LA Cagematch run, they held the record for longest streak)
Diamond Lion (musical improv, UCB LA/NY)
Mother reunion (Scot Armstrong, Tara Copeland, Jon Daly, James Eason, Jesse Falcon, Jason Mantzoukas, Doug Moe, Jessica St. Clair, and Christine Walters): Mother's been one of my favorite groups ever. They were great improvisors and actors, but as a team, they were even more amazing. They started out as a UCB-NY Harold team in 1999 & moved up to weekly weekend shows and had one of the longest runs at the theater before most of them to LA.
Bare (Chicago;
http://fuzzyco.com/bare/main.html): Fuzzy Gerdes & Shaun Himmerick are still one of the best two-person improv shows I've ever seen. They don't perform out a lot but they'd put on a great show.
Messing With a Friend (Susan Messing + a special guest)
Children of a Lesser God (Susan Messing, Rachael Mason, Emily Candini, Kate Duffy)