Oh so many.
Sean O'Haire - Very talented guy. Never given a time to shine in WWE. They push the hell out of Lashley when he has little mic skills, but give Sean no time to develop his.
Matt Morgan - The best big man to come through WWE this millenium. Oh, and he was legitly big. No steroids. They had him destroy the Big Show on a few separate occasions, then they go and release him. What?! And he was adequate on the mic. All he needed was a good gimmick and he could've been the big man they so desperately want Khali and Snitsky to be.
Tony Mamaluke - They brought him back and put him to absolutely no use in the new ECW. Instead of releasing him, they should've put him on Smackdown where he could add some different styles to the Cruiserweight division. He was one of the best cruiserweights I've seen. Need proof? Look up the match between him, Shark Boy, AJ Styles, Low Ki, and a couple of others from like 2001.
Billy Kidman - He was done with the cruiserweight scene. But he would've been great in the US or Intercontinental division.
Kid Kash - I shouldn't have to explain this. But he would've been great in ECW instead of tagging in a lame gimmick with Jamie Noble (who is criminally underused).
Flash Funk - I only say this because he could've been a great jobber to the stars. He could've been to Smackdown what Val Venis is to Raw.
Vito - "Hey, this guy is really over with the fans, we've got him on a winning streak and he'll do whatever we tell him to. We better fire him quick." They should've kept him, left him in a dress, but make him "snap" and become a crazed, ass-kicking machine.
K-Kwik/Ron "The Truth" Killings - I am so hoping that they realize what they let go and sign him back now that he's walked out of TNA. One of the best all around wrestlers working today (he can do high flying, he's a mat technicians, he can go hardcore, and he can do power). Plus, you've got an almost instant feud with the golden boy, John Cena, since WWE likes to bring real life beefs into the show.
Joey Mercury - Drug problems withstanding, he was the talent behind MNM. Sure, Nitro has gotten better since going solo, but Mercury held the team together.
Orlando Jordan - Could've been a great worker in the future. Would've been a good fit for a tag team with Elijah Burke.