It's astonishing how many cars were left off after waiting so long for GT5 to be released. Some off the top of my head- some are no-brainers, others are personal faves:
AMC
Javelin SST, AMX
BMW
E30 M3
E36 M3 Euro-spec
850 CSi
E39 M5, 540i
Buick
GSX 455 Stage 1
Bugatti
EB110SS (or Dauer)
Cadillac
CTS-V
Chevrolet
70 Camaro Z28, SS
1993-1995 Corvette ZR1
1987-1990 Camaro IROC-Z L98 5.7
1992 Camaro Z28 Heritage Edition
Trailblazer SS
Chrysler
Conquest TSi
Dodge
92-93 Daytona IROC RT Turbo III, HT 2.5
91-93 Stealth RT Twin Turbo
94-95 Stealth RT Twin Turbo
Eagle
97-98 Talon TSi AWD
Ferrari
288 GTO
512 TR
456 GTM
575M
599 GTO
F355
365 Daytona
Ford
84-86 Mustang SVO
87-93 Mustang GT, LX hatchback, LX notchback
93 Mustang Cobra, Cobra R
94-98 Mustang Cobra ohv 5.0
96-98 Mustang Cobra dohc 4.6
03-04 Cobra SVT
93-95 Thunderbird SC
69-70 Mustang Mach 1/Boss 302, 351, 428, 429
Torino Cobra Jet
GMC
Syclone, Typhoon
Lamborghini
Diablo
Mercury
Cyclone Spoiler
Mitsubishi
90-91 Eclipse GSX
92-94 Eclipse GSX
95-96 Eclipse GSX
97-99 Eclipse GSX
Starion ESi
Oldsmobile
Cutlass 442
Pontiac
68-70 GTO
70 Formula, Trans Am
73-74 Formula 455 SD, Trans Am 455 SD
77-81 Trans Am
88 Fiero GT
89 Turbo Trans Am
87-90 Formula 350, Trans Am, GTA
91-92 Formula 350, Trans Am, GTA
91-92 Firehawk
93-97 Formula, Trans Am, Firehawk
98-02 Formula, Trans Am, Firehawk
97-03 Grand Prix GTP
SAAB
9-3 Viggen, 900 Turbo, 08 Turbo X, Aero X concept, 9-2X Aero, 9-7X Aero
Volvo
Volvo S60R, V70R, 850R/T-5R, C70