The Penguins signed defensemen Matt Carkner, formerly of the San Jose Sharks organization, to a one year deal at or near the minimum ($450,000). Carkner has only played one game at the NHL level (it was last season), but has had some success in the minors.
I'd guess this is someone that Shero believes could stretch beyond his current boundaries. He's not that old yet either - just 25. He has size (6'4") and toughness (over 200 penalty minutes last season in the minors) but also put up some points last year for the Cleveland Barons (10 + 21). If he can play his way onto the roster, then great. Otherwise, he'll provide some depth and eat up some minutes in Wilkes-Barre, where their roster is still kind of sparse. I hope he can play on the top level, but it's not usually a good sign when someone is the career leader in games played for an AHL franchise - it means he's been there for awhile.