I was going to put Jonathan Sanchez until his recent trade, but Matt Cain could definitely be traded. The Giants have one of the best rotations in baseball, led by Lincecum, Cain, Bumgarner and Vogelsong.
In return for Matt Cain, the Giants would get two things: a great offensive prospect and payroll relief.
For example, if the Giants decided to trade Cain to the Yankees they would get Jesus Montero in return on top of a pitcher like Dellin Betances or Phil Hughes. Jesus Montero would be a menace in the middle of the lineup. If the Giants took Betances he would replace Zach Wheeler (now with the Mets) as the Giants' No. 1 pitching prospect, but if they took Hughes he would instantly become their No. 4 starter with a high upside.
The last Yankees' top pitching prospect to go bust in New York and get traded to the NL West was Ian Kennedy and he worked out very well.
The $15 million opened up by Cain's absence could be used to sign a top free agent, such as Jose Reyes. Their new lineup would feature players like Jesus Montero, Brandon Belt, Buster Posey, Jose Reyes and Carlos Beltran (if he is re-signed).