Brett Myers: He could be a guy you get cheaper than Lilly and hes having a better year than him too (3.71 FIP vs Lilly's 4.70 FIP). Myers has always had decent stuff but the homeruns have really plagued him despite strong groundball rates; this year however, hes done a terrific job of suppressing them (0.77 HR/9), but should longball problems return, he'd certainly be aided by the spacious Citi Field. He has a mutual option at $8M for next season, which could be a nice bargain should he and the Mets agree to pick it up.
Ted Lilly: While he might not be having the best year peripheral-wise (4.70 FIP), he has a strong recent track record (2007, 2008, 2009) which makes one believe he'll be better in the 2nd half. Another thing of interest about Lilly is that hes a potential Type-A free agent after the season, which means, assuming the Mets offer arbitration, they could end up collecting a pair of draft picks if he signs elsewhere; on the flip-side, if he accepts arbitration (which I suppose is possible), then the Mets would have him back for next season, and that might not be the worst thing in the world, especially if he were to pitch better in the 2nd half of this season.
Ben Sheets: Hes kind of in the same boat as Lilly in that his peripherals haven't been strong (4.75 FIP), except the primary differences are: 1) he probably wouldn't cost as much as Lilly in a trade, and, 2) he won't bring back any draft pick compensation after the season. He might have the best upside of this group, but a return to his vintage form doesn't seem likely.
Kevin Millwood: People probably want to kill this idea with the way hes pitched (and hes on the DL too now, I think), but he'd actually be a decent fit in Citi Field. His 4.33 xFIP suggests hes not pitching as poorly as his ERA (5.77) or FIP (5.03) would indicate; his main issue here is that hes getting killed in that bandbox in Baltimore, and the splits back it up: HR/9 at home - 2.38, HR/9 on road - 0.88; that's quite a difference. A move to Citi Field and getting out of the AL East would do him wonders. He wouldn't cost anything either, as the Orioles would probably happy to get a bag of balls in return for him. Millwood projects as a potential Type-B free agent, though I suppose offering him arbitration after the season might be a bit questionable. Nonetheless, he could be a good pickup for the stretch run, and he'd certainly cost the least of this group.
Jake Westbrook: I originally opposed him, but I've come around to believing he'd be a fine addition. In a nutshell: hes not going to miss many bats but he'll throw strikes and get a lot of groundballs. A move to the NL and Citi Field would definitely be beneficial and probably give a slight boost to his overall peripherals. Westbrook is a free agent after the season but he won't bring back any draft pick compensation as he doesn't project to be a Type-A or B free agent.