For those that didn't see it, I've set up an attempt at a comprehensive list of secondary appearances on BB by former contestants. It's on page six.

Here's the season ranking at the moment, determined by the number of seasons a given player appeared in outside of their original game:
BB2 - 21 (Will (x4), Bunky/Mike (x3), Hardy/Nicole/Monica/Shannon (x2), Autumn/Sheryl/Kent)
BB3 - 17 (Amy/Danielle/Lisa/Josh/Marcellas (x2), Gerry/Roddy/Chiara/Tonya/Eric/Lori/Jason)
BB5 - 16 (Jase (x3), Diane/Nakomis/Cowboy/Holly (x2), Scott/Marvin/Adria/Drew/Natalie)
BB4 - 13 (Jun (x3), Allison/Erika/Jack (x2), Dana/Robert/Justin/Jee)
BB6 - 9 (Janelle (x3), Kaysar/James/Howie/Ivette/April/Eric)
BB8 - 6 (Jen (x2), Dick/Eric/Jessica/Daniele)
BB7 (post-season) - 5 (Janelle (x2), George/Mike/Will)
BB1 - 3 (George (x2), Josh)
BB9 - 2 (Matt, Sheila)
Obviously, the "Where Are They Now?" segments contributed greatly to the contestants of seasons 2-5 being continually rammed down our throats. Surprisingly, not so much for BB6 considering people wouldn't shut about that season for two years.