Lopez is a sidearming lefty reliever who was excellent for Boston in 2006-2008. Low WHIP and HR rate, but also low strikeout rate and high walk rate. In Boston's pen he was the quintessential LOOGY, but may be asked to do a little more with the Bucs.
This is not the typical power arm that NH has been acquiring of late and from that perspective I like the different look that he brings to the pen.
Terms to follow.
UPDATE by Charlie: Cool. I highlighted Lopez as one of the more interesting minor league free agents this winter. He's not terribly old and he was very effective from 2006-2008, so the $775,000 plus incentives the Pirates will pay him looks like a great gamble. He has his flaws (in particular a not-so-great K:BB ratio), but he'll benefit from a move to the easier league, and as a lefty who's very effective against lefties, he fills an obvious need.