Lopez, 33, has two impressive numbers to his name this year: a 2.79 ERA and a 60.2% groundball rate. Otherwise he's struck out few and walked many overall. His splits show that he's somewhat effective against lefties and a poor choice against righties. He signed a one-year deal with Pittsburgh before the season, and is still owed $287K this season. He is under team control as an arbitration-eligible player next year.
Martinez, a 27-year-old righty, posted a 3.32 ERA, 7.2 K/9, and 2.9 BB/9 with six home runs allowed in 81.3 Triple A innings this year. Baseball America ranked him 30th among Giants prospects heading into the '09 season. They called him "a groundball machine" with a "sinking 86-88 mph fastball, a true curveball and a plus changeup consistently down in the strike zone." Martinez has yet to figure things out in his 41 career Major League innings, but he has retained his ability to generate grounders.
Bowker, a 27-year-old outfielder and first baseman, hasn't done much in the Majors aside from a less-than-terrible '08 rookie season. He has managed to rake in the Pacific Coast League after multiple stints there, for what it's worth. Baseball America ranked him ninth among Giants prospects before the '08 season, saying "he has strong hands and can hit good fastballs."
Dejan Kovacevic of The Pittsburgh Post-Gazette tweeted that Lopez was on the verge of being dealt while Andrew Baggarly of The Mercury News later confirmed the deal and added details (all Twitter links).
Mike Axisa contributed to this post.