Despite the recent acquisition of quarterback Matt Flynn, the Raiders aren't sitting on their hands in relation to the quarterback position. Flynn was brought into Seattle to be the probable starting quarterback there, but Russell Wilson proved he, not Flynn, deserved to start. With the same scenario possible, but not probable, in Oakland, the Raiders are still looking to add depth at the position.
Ian Rapoport of NFL Network reports that the Raiders have worked out Vince Young and Troy Smith. The Raiders passed on signing Seneca Wallace. Notably, all three of those quarterbacks are very mobile style passers. Terrelle Pryor, currently one of the Raiders' two quarterbacks, is also that style of passer.
Vince Young and Troy Smith are just looking for futures in the NFL. It seems likely that the Raiders will sign one of them. The odds are that Vince Young gets a chance with the Raiders. He's had some ups in his career, but the low points have certainly outweighed those highlights. Young won games as a starting quarterback, and if he can get his mind in a good place, he could give Matt Flynn and Terrelle Pryor a run for the Raiders starting job.
There's been no word on how the workouts went for Young and Smith. Troy Smith has always been a backup quarterback in the NFL, partially due to his short-for-a-quarterback height of six feet (on a good day).
Regardless of who the Raiders do decide to sign out of free agency, if anyone at all, they certainly seem interested in mobile quarterbacks. The AFC West is currently home to three starting quarterbacks that don't run particularly well. Philip Rivers and Alex Smith are somewhat mobile, but Peyton Manning is a statue in the backfield. Those teams aren't used to seeing quick quarterbacks, and that could give the Raiders an advantage if they start such a player at the quarterback position.