The Rays have signed first round pick Mikie Mahtook, according to the St. Petersburg Times (on Twitter). It's a $1.15MM deal, according to Jim Callis of Baseball America (on Twitter). Mahtook, 21, is a right-handed hitting outfielder who played his college ball at LSU. The Rays, who had ten of the top 60 picks in the draft, used their second selection (31st overall) on Mahtook.
You can keep track of which top picks have signed with MLBTR's list.