One of my favorite sleeper prospects for 2013 is Jesse Hahn of the Tampa Bay Rays. Here is what I wrote about him in the 2013 Baseball Prospect Book, which explains why.
Jesse Hahn, RHP, Tampa Bay Rays
Bats: R Throws: R HT: 6-5 WT: 185 DOB: July 30, 1989
2011: Grade C; 2012: Grade C
SLEEPER ALERT!! Virginia Tech's Jesse Hahn was going to be a first round pick in 2010 until he hurt his elbow late in the spring, dropping him to the sixth round. He missed 2011 recovering from Tommy John surgery, but he came back in 2012 and was very effective in the New York-Penn League, working at 92-94 MPH and hitting 98-99 at times, which was even better than he threw in college. His curveball, slider, and changeup had some rust to work off in his first few starts, but by the end of the season he looked like his old self, dominating the competition with a combination of strikeouts and grounders (2.88 GO/AO). Rays fans are aware of him, but he might be forgotten by the broader national audience. He shouldn't be, so I'm sticking the Sleeper tag on him. A healthy Hahn could become a number three starter, perhaps more. Grade C+
ADDITIONAL COMMENT: I'm not alone in this; Yossi Feins over at DRaysBay also sees Hahn as a breakout/sleeper candidate.
Hahn posted a 2.77 ERA with a 55/15 K/BB ratio in 52 innings for Hudson Valley in the New York-Penn League last summer, allowing a mere 38 hits, zero homers, with the aforementioned 2.88 GO/AO ratio. His mechanics are reportedly improved from what they were pre-injury, which enhances his command and should (one hopes) help him avoid further injury.
While seeing Hahn in the majors in 2013 is a real longshot, it would not surprise me at all if he ranked among the elite pitching prospects in the game entering '14, even at age 24.