This thread got way off on a tangent...so let's break it down.
Hidden cars for NTSC versions (requires gameshark):
Mercedes CLK Race Car (a.k.a. CLK-DTM)
VW Polo
Shelby GT500 KR
Japanese-spec NSXs (like '92 Type R, mostly same as Acuras)
Japanese-spec Integra Type R
Japanese-spec Integra Type R 98 spec
an extra Shelby GT350 (same as lot version, though)
an extra Toyota 2000GT (same as lot version, though)
Peugeot 406 sedan (same, but race mod has Esso stickers)
Peugeot 206 rally car (same, but race mod has Esso stickers)
[R] Toyota GT-One '98 (same, but race mod has Esso stickers)
[R] Toyota GT-One '99 (same, but race mod has Esso stickers)
Body-only cars (just a body, not a real car)
Ford GT40 Mark III
Subaru Rex supercharged
Nissan 200SX (exists on Japanese versions of game)
Nissan Fairlady Z (exists on Japanese versions of game)
Nissan Primeras (same as Inifinti G20s)
Cars that are impossible to find/not in game:
McLaren F1
Mercedes-Benz CLK-GTR
any Ferrari
any Lamborghini
any Porsche
...and anything else that is not on this website.