Here's a small list of pitchers that could potentially be moved in an August trade, from MLBTR:
Blue Jays: Darren Oliver (L)
Twins: Brian Duensing (L)
White Sox: Matt Lindstrom ®
Mariners: Oliver Perez (L)
Angels: Joe Blanton ®
Astros: Wesley Wright (L), Travis Blackley (L)
Phillies: Jonathan Papelbon ®
Mets: David Aardsma ®, Scott Atchison ®, Pedro Feliciano (L), LaTroy Hawkins ®
Marlins: Chad Qualls ®, Ryan Webb ®
Cubs: Kevin Gregg ®, Matt Guerrier ®, James Russell (L)
Brewers: John Axford ®, Burke Badenhop ®, Mike Gonzalez (L)
Padres: Huston Street ®, Tim Stauffer ®
Rockies: Matt Belisle ®, Manny Corpas ®, Wilton Lopez ®, Josh Outman (L), Rafael Betancourt ®
Giants: Javier Lopez (L)
Out of those listed, I'd probably be interested in Duensing and Lindstrom, with a guy like Street being a decent option as well. Seems we have a plethora of left handed relief as it sits right now, but Duensing has been pretty solid in the past and has the ability to go long as well, having started games before with the Twins. Not sure what it would take or even if the Sox would be interested, but he wouldn't be a bad BP addition I think.