Hefty lefty Brad Kilby was dominant as a September call-up last year, posting a 0.53 ERA (and 1.99 tERA) in 17 innings, with a 20/3 K/BB ratio.
Kilby has solid stuff, with a low-90's moving fastball and low-80's sweeping slider. Both pitches grade out as solid-average.
The secret to his consistent minor (and major) league success is a tricky delivery in which Kilby removes the ball from his glove early, draws his left arm back to his left side, and does a sort of pump-fake, before finally coming forward and delivering the pitch.
Kilby's not the most in-shape person, so he uses his girth to his advantage; his delivery keeps the ball hidden behind his stomach until the last second, until the ball leaps out. His 92 mph fastballs can look like 97 to a hitter.
(You can view Kilby's odd delivery by clicking the video on the left at http://oakland.athletics.mlb.com/search/media.jsp?player_id=489020)
Effective against both lefties and righties, Kilby should take over as the primary lefty in the Athletics' pen this year. He should be very good.