A fun trip down memory lane:
Nick Buoniconti, Phil Olsen, Jim Plunkett, Russ Francis, Bob Golic, Steve McMichael, Irving Fryar, Rich Gannon, Ray Agnew, Chris Singleton, Kevin Turner, Curtis Martin, Christian Peter, Greg Spires, Tebucky Jones, Sean Morey, David Thomas.
Ty Law is debatable. He did have a career high of 10 interceptions the year he left the Pats, but overall I would say he had more success in New England. Seymour does not belong on the list, he had far more success in NE. Same goes for Deion Branch and David Givens. Terry Glenn had some good seasons but was not as successful as he was in NE. Andruzzi did start for two seasons after leaving NE, but he started 72 games in five years with the Pats. Ellis Hobbs and Asante Samuel may be playing but I don't see how they would be considered to be more successful at this time. Daniel Graham is borderline, I'd be slightly inclined to include him as having better years in Denver than in NE. Time will tell with Matt Cassell and Ben Watson.
I suppose you could include Kevin O'Connell on that list, as he went on to become captain of his new team ...
edit: forgot the pride of East Boston; Jermaine Wiggins should be included. Though he'll be far more remembered for the 2001 playoffs, overall he was more successful with the Vikings: started 24 games with 186 receptions for 1659 yards and 6 TD in Minnesota, compared to starting 8 games for the Pats with 30 catches for 336 yards and 5 TD with the Pats.