To be honest, I think Federer prety much has Wimbledon in the bag already. OK, so he lost to Safin, but he still had match point in that one, and his game is Sampras-like on grass. With that serve, Roddick is a definite factor and he would have won Wimbledon the last two years had it not been for Federer. Roddick needs to start getting into the net more to cut off the points before they get to the long baseline rallies, and this would especially help him on grass.
Grass is possibly Hewitt's best surface, so he can be considered a contender; the quickness of the surface and the low bounce allows his flat groundstrokes to get more bite than they would on clay or a slow meduim-bouncing hardcourt like at the AO. However, it should be pointed out that when Hewitt won Wimbledon 2002, he had a joke of a draw; playing only 1 Top 10 player, and that was Henman in the semis who had a injured shoulder and couldn't serve properly to save his life. Had Agassi received that draw, he probably would have won Wimbledon that year, Hewitt's opponents were all baseliners: Shalkcen, Nalbandian etc.
Tim Henman asserts that the courts at Wimbledon have slowed down substantially over the last 5 years or so, and that helps guys like Agassi, Hewitt and Nalbandian, but hinders Tim, although there is now little doubt that his best chances of ending the long British wait are behind him. I also wouldn't count out Mario Ancic, the last guy to beat Federer at Wimbledon (first round in 2002).
It would be a big long shot for Agassi to triumph again at Wimbeldon, but if he's serving well and, say, Federer withdraws due to injury, it is not totally out of the question. It would be on of the biggest stories in sport ever as Agassi would be winning Wimbledon 13 (!) years after his first title there, which would be the longest gap ever between Wimbledon wins.
However, it's been two years since Agassi even played on grass, apart from his first round loss to Andreev at Queens last year. He then pulled out of Wimbledon citing a wrist injury, when really it was due to a complete lack of confidence. Agassi knew that, due to his dip in form, there was a strong possibility that he would lose early on at Wimbledon, and he believed that it was not worth taking that risk because the calls for him to retire would have grown louder than ever before. He skipped the post-match press conference at Queens last year because he knew all the questions would be retirement-related.