dunno how many of these have been mentioned but here you go:
Iker Muniain - ridiculously attainable at 16 from Bilbao, got him for £500k
Micah Richards - kinda obvious, hard to get from Man City though. Great CB or RB he's fast and physical
Domenico Criscito - Awesome CB from Genoa, also good at LB.
Emiliano Insua - Give him a few years, great left back. Liverpool won't sell him cheap though.
Marek Hamsik - starts at 85, but I managed to sign him for £9m when he was 24, rated 91 and 'chances were zero' (my scout was wrong!) from Napoli. He can do anything.
Anderson - United won't sell, but if you get your ranking up you can get him after about 3 seasons. He peaks late, so it's ok.
Sergio Busquets - got transfer listed by Barca in my game, was a bargain (so was Pedro, but Busquets is just class on grass)
Ever Banega - starts age 20, too damn good as a DMF can score from distance too.
Joe Hart/Rui Patricio - great young keepers. Also look out for the Youth team regen of Akinfeev, around season 2.
Gareth Bale - becomes a brilliant left-back and dead-ball option, starts age 19 try and get him from Spurs.
Tremoulinas - Bordeaux left back, rated 81 at age 23 in third season.