What happened to these guys?

I always find that players seemed to disappear a lot sort of like Shawn Alexandra who hasn't been that good lately, i don't know what happened to Chad Jackson of the Patriots, As for Dillon, Martin, Barber and Troy Brown they all retired. I also wonder where Donte Culppper has been he use to be a really good QB, Priest Holmes had too many injures, Jake plummer must be a back up somewhere i have no clue. I guess alot of players seem to retire with out big notice