Cardinals RHP Zach Petrick (undrafted, 2012) has been stellar so far in the minors and looks to be another late-round/undrafted success story like current major league relievers Trevor Rosenthal (21st round, 2009) and Kevin Siegrist (41st round, 2008).
At 23 (soon to be 24), he's old for the leagues he's played in, but the numbers are outstanding. He has posted a 0.83 ERA, 12.7 K/9, and 5.75 K/BB in 16 games with low-A Peoria, an 0.27 ERA, 8.6 K/9, and 8.00 K/BB in 10 games with high-A Palm Beach, and has a 3.09 ERA and 8.5 K/9 in two starts with double-A Springfield. The Northwestern Ohio (NAIA school) graduate most likely won't contribute to the major league roster this season, but he'll be an interesting player to watch in 2014 with guys like Michael Wacha and Carlos Martinez more than likely exhausting their prospect eligibility.
7/25/13 edit: Regarding Petrick, Law's impressed enough that he sees him as a possible back-end starter type.