A few spring to mind - Rosler, Quinn, Hinchcliffe, Walsh, Coton and Kinky of course. More up to date Anelka and Dunne if he goes, and on a sadder note Marc Vivien Foe.
But the biggest one for me in actual transfers out, weirdly some may say, would be David White. I started watching city in '87 so it was just as the Lakey-era of players were coming through and it was really exciting and something to be proud of. As a kid I watched us thump team after team it seemed like, and David White was a huge part of that. When Leeds robbed us of him I felt like it was the end of something, and an admission that they were bigger than us. Ok, so he was a kick and run merchant and no Kinkladze on the ball, but he was the guy who stuck 'em in every game I watched as a kid. I was only about 9 at the time and I went to the Meadowhall Centre with my family not long after he moved and saw him there - I just stood there looking at my ex-hero wondering how he could play for Leeds after all he had done for City, knowing that the Leeds scumbags would never appreciate him as he had been at City.
And for the record, David White scored our first Premiership goal. I still maintain that if we'd kept him and he'd avoided injury, Beagrie on the left and White on the right with Quinn and Walsh or Rosler upfront would have been top notch and changed our history. With the addition of a couple of defenders maybe...
Hmm. quick visit to wikipedia reveals that:
Later it would transpire that White had requested a transfer when Brian Horton took over at the club, fearing a break up of the team he had been a part of under Peter Reid. White would later admit to regretting his rashness and that he never really gave Horton a chance to change his mind.
Never knew that. Oh David, tut tut.