Tougher challenge than it would seem at first. Others here have mentioned my first choices, but I would include Crosby, Stills, Nash, and Young, which included members of the Byrds, Buffalo Springfield, and the Hollies. Derek and the Dominoes also included a member of the Allman Brothers. Jeff Beck was in Beck, Bogert, and Appice (with a member of Vanilla Fudge), and Jimmy Page was in another group besides the Yardbirds, Led something or other.

Oddly enough, other than Clapton's enterprises (and I'm not a big Clapton fan), I tend to like only one of each from all of these combos.