Cooper was the first 1st rounder to sign, getting a reported $1.5MM signing bonus.
Here's the complete list.
The most notable players not to sign are 3rd and 4th round picks RHP Andrew Liebel and 3B Robert Sobolewski (who as Marc points out, can't sign until after his team is out of the CWS).
Two players with injury concerns (who fell in the draft because of it), OF Eric Thames and C Antonio Jiminez both signed out of the 7th and 9th rounds respectively.