There will not be too many top-tier starting pitchers on the trade market this summer. Zack Greinke and Cole Hamels are names that there will be a lot of speculation about, but after them the talent level drops off.
Jason Churchill of ESPN (Insider required) has suggested that the Tampa Bay Rays could look to deal James Shields this summer in an effort to push themselves towards the playoffs. His logic is that the Rays have starters that can fill in for him and that they can address a need while picking up a prospect or two if they trade Shields.
While a trade like this seems to be very unlikely, there are certainly a number of teams that would be willing to pay what the Rays want in return for Shields.