If you're looking for a rookie who's stood out at multiple practices, it's Green Bay Packers WR Randall Cobb. He made a big play at the Family Night practice at Lambeau Field. From Jason Wilde at ESPN Milwaukee:
Working with the No. 1 offense in a three-receiver set because starter Greg Jennings sat out, Cobb reeled in a 40-yard back-shoulder pass from quarterback Aaron Rodgers on a second-and-10 play for the night’s longest gain.
He went onto mention that he and Rodgers appeared to be on the same "wavelength" once Erik Walden jumped offsides. And I'm impressed by the little things that make a big difference: he didn't drop it.
Cobb is probably the player to watch this preseason, and it might only be possible to watch him in the preseason if he gets caught at the bottom of a deep depth chart when the regular season begins.