The Pirates signed several different players to minor league and non-roster invite contracts. This is, of course, the lowest risk type of signing you can have, so you don't expect to find the next Albert Pujols or Roy Halladay in these kinds of signings. The question is whether or not you can find players of value to sign to these kind of contracts. First, Jake Fox, who has a -.8 career WAR in 534 PA, which would be about a full season worth of plate appearances. Most of this is his poor defense, a -.6 D-WAR, with hardly any real position, playing everything but SS, 2nd, and CF. There is a reason he plays all those positions, he is below average at every single one of them according to Range Factor. But his hitting is also below replacement level at a -.2 O-WAR. His OBP is just .288, with a OPS + of 87, and a Secondary Average of .243. He does have an Isolated Slugging of .188, which is decent but he is a way below average walker and has an offensive winning percentage of .413.
Next is Brandon Boggs. In 382 PA, he has an even 0 WAR. He plays every outfield position, but under league average according to Range Factor and Fielding Percentage. His OBP is .315, with an OPS + of 83. He has a good Secondary Average of .328, with 4.4 Runs Created per game (but just a .449 Offensive Winning Percentage). He is a great walker at 12.8%, an average home run hitter and an above average extra base hitter. He sees a very high Pit/PA of 4.33, giving him a PPS of 102.49, which is very good. There is certainly value there.
They also added Kyle Cofield, but I profiled him when he was released by the White Sox.
Shairon Martis had 2 years of experience with the Nationals, throwing 106.1 innings, basically all as a starter. He has a WAR of .1, PE of 4.38, adjusted PE of 4.59, and SIERA of 5.49.
Stefan Welch has been exclusively a minor league hitter, spending 5 different seasons in different levels of A ball. He has had a .337 OBP, 1.76 04S, and 2.99 PAPP. He walks just 7.6% of the time, and homers just 1.9%. A profoundly mediocre A ball player.
Lastly, Jose Diaz has pitched just 7.2 major league innings, and at least as far as I can see, didn't pitch in 2011.