Brees was the biggest and obviously a unique situation, then Grubbs, then Sproles. Past that we traded for Shockey and Jeff Faine and that's really it. Then you're in David Thomas, Jonathon Goodwin, Mark Brunell territory.
Grubbs and Sproles were direct responses to high level losses (Nicks and Bush). Really only the Jeremy Shockey trade (and Brees to begin with) has been the only time Payton has looked outside for a clear roster upgrade on offense.