Brent Grimes I believe is scheduled to be a free agent. That's an out-of-the-box, ready-to-play-right-now kind of guy that would give you at the very least solid play right away in Year 1 with the possibility of better than that in Year 2, as opposed to a rookie which you wouldn't expect that until Year 3 (if ever). Cary Williams is set to be a free agent and that would probably be a tremendous signing. Both guys would be "break the bank" signings, IMO.
Rashean Mathis would be a great veteran presence if you don't think he's done physically (he's playing well this year). He could play solid football for you right away. Chris Houston will be looking to cash in a little bit on his success in Detroit, although IMO he won't be a "break the bank" signing like the two guys above (he made $3 million this year). He's a great example of how sometimes you just need to be patient with corners in their development. But also perfect example of how even if you draft one destined to be good, that doesn't mean YOU will be the team reaping the rewards for that. You might take a look at Michael Jenkins if you think you can re-claim him and he's a guy that will be one of those guys long in the development but worth it. Tracy Porter is playing surprisingly well for Denver and should be available once again next off season. He struggled in the past, but see my overriding theme above about corner development and how it takes time.
At the very low end for impact guys, Brice McCain is available and he played with Sean Smith at Utah. Something about Utah's defenders seems to be favored by Miami, and he's good enough to actually play, not embarrass himself, and potentially play well. But I wouldn't necessarily be convinced we're getting better than Richard Marshall, with him. Guys like Sheldon Brown and Cedric Griffin won't look attractive because of their age and because you know at this point that they're not going to be shut down corners, but they could still be very solid players that hold their own on championship caliber defenses, and that's exactly what you CAN'T guaranteed from any rookie.
If we're talking slot corners (and that's been Richard Marshall's role) then we just played against a team that has a pretty good one coming up as a free agent. Michael Adams pretty much replaced Richard Marshall in the slot during the 2011 season, and I thought he played better. I thought this was the main thing that contributed to Marshall's move to safety. Adams has been hurt since the first game and that opened the door for hot shot rookie 3rd rounder Jamell Fleming to come out on the wing with I believe William Gay moving in to the slot. Fleming is playing well there, though Gay is not necessarily performing well as a combo player.