The original Banshee and Rumpo ( renamed as 'Burrito' in latter games) from GTA III;
Cheetah, Infernus and Deluxo (would be great if the De Lorean DMC-12 is brought straight into GTA IV, instead of it :p) and Sabre Turbo from GTA Vice City.
Bullet and Buccaneer from GTA San Andreas.
There could be some more that I am missing
Huge thanks to K-evin
for this signature.