I'm surprised everyone has been thinking about the Mariners suggestion. But if they trade Doubront/Bowden/Dice-k, it cleans up the congestion for the 5th pitching slot, the Mariners get 3 starters with potential (plus more, likely) and a Sox rotation of Lester, Lee, Buchholz, Beckett, Lackey is monstrous. When Cliff Lee was traded to the Mariners, it really sounded like he was tired of getting shipped around, so I'd bet he'd be easy to lock up if on a contending team.
Another idea is trading Ellsbury in a big bat trade, and going all out for Crawford next year. Ellsbury is a great target for a team that wants to rebuild for next year. He's a good, cheap player who is under control for a while, so while he's valuable for the future, he's been a hassle for the Sox this season. Crawford fills all the needs that Ellsbury did, so they could use Ellsbury as a chip to pick up that key piece.