Melancon is a 26-year old right-handed relief pitcher who recorded 20 saves for the Astros last season. He's a former Yankee who was included in the deal for Lance Berkman.
Here's a brief scouting report:
Melancon, who turns 27 next March, rode his dinger-limiting repertoire of curveballs, changeups and fastballs with heavy movement to 20 saves in 25 chances, with a 2.78 ERA, a 7.99 K/9 and a 3.15 BB/.9 this past season. The righty doesn't generate quite as many swings and misses as the prototypical closer and so is a little more at the mercy of the BABIP gods.
Here's hoping he can handle the big market pressure.
Update: The ProJo's Brian MacPherson has more info on Melancon and the deal.