From the Prospect Blog at Baseball America (no subscription necessary), here is the list of the hardest throwing left-handed starters in the 2008 Prospect Book. Given the dearth of hard-trhowing lefties, this list could be considered a premium sleeper one in deep keeper leagues.
For me, the Washington Nationals' Ross Detwiler drew my attention. He got to pitch for the Natspos in September so should be considered likely to see time in 2008. Normally, I'd let him slide, as I do with most starting pitchers on bad teams, but his ranking amongst the hardest throwing lefties, has me reconsidering.
A "#" means the pitcher mised 2007 with an injury or will miss 2008.