Well, first off, I was sorry to hear this news and I feel for you fans, this must be a real choker. I don't blame anyone for retiring if their heart isn't in it but the timing is terrible on his part.
Still, the possibilities if the Dolphins want to bring in someone could be something like this:
Trung Candiate (FA)
Ladell Betts (trade from Redskins, would probably cost about a 4th rounder next year)
Either Blaylock or Larry Johnson from the Chiefs (trade, can't guess what it would cost)
One of Minnasotas backs (again, trade)
Lamont Jordon or Travis Henry in a trade, problem is that they are division rivals and may be very reluctant to do business, plus in Jordans case, he could be about to replace Curtis Martin.
Bears could be willing to trade A-Train.
Broncos may be willing to trade Quentin Griffen if Bell and Hearst look good in camp.
Maurice Morris of the Seahawks (again, trade).
James Stewart of the Lions!
Those are just of the top of my head, the Phins have to be careful in any trade, teams will look to rip them off knowing the situation the Phins are in, may be best to go with a FA who has something to prove plus Minor who knows the system and can play a bit.
As I say, bad news for you guys, Williams was a great player, the league needs as many great players as it can get. Hopefully for you guys this brings your team closer together and makes them want to prove they can do it without Williams. A team with a chip on its shoulder can be dangerous!