For Siddley and others who don't have access, here's a recently-updated list of cars that are (or hopefully soon will be) licensed for use in the game. None of these are 100% completed, although many are currently drivable in an excellent state graphically, with varying degrees of completeness on suspension/tyres/aero/damage modelling. Some are very new announcements, e.g. most of the Audis, and haven't been started yet. Some of them may be held back for themed DLC packs, e.g. some historic cars and tracks - although this is purely supposition on my part, as the DLC plan hasn't been released yet, and SMS have been known to surprise us with near-completed cars and tracks out of the blue.
(There's still hope that cars from other marques will make it into the game before release. The likes of Porsche and Ferrari appear to be out of reach, due to restrictive agreements those manufacturers have with Microsoft and Electronic Arts respectively. The most common requests seem to be for various Aston, Nissan and Chevrolet road and race cars. However, due to the apparently-delicate nature of license negotiations, we don't have a clue what licenses SMS are actually seeking, let alone which they may be close to getting agreement on.)
A lot of this info is available at http://www.wmdportal.com/.
Ariel Atom 3 (2011)
Ariel Atom Mugen (2011)
Ariel Atom 500 V8 (2011)
Audi 90 IMSA GTO (1989)
Audi A1 Quattro (2012)
Audi R8 V10 Plus (2013)
Audi R8 LMP900 (2002)
Audi R18 TDI LMP1 (2011)
Audi R18 e-Tron Quattro (2014)
Audi R8 LMS Ultra GT3 (2013)
BAC Mono (2011)
Bentley Speed 8 LMGTP (2003)
Bentley Continental GT3 (2013)
BMW 1M Coupé E82 (2012)
BMW 320 Turbo Group 5 (1978)
BMW 320TC BTCC (2012)
BMW 328 Touring Coupe (1940)
BMW F1.08 (2008)
BMW M1 Procar (1981)
BMW M3 E30 GroupA (1991)
BMW M3 E92 GT (2012)
BMW M3 E92 GT4 (2012)
BMW V12 LMR (1999)
BMW Z4 GT3 (2012)
Caterham Classic
Caterham Superlight R500
Caterham SP/300.R
Ford Escort RS1600 (1972)
Ford Falcon FG V8 Supercar (2013)
Ford Focus RS (2009)
Ford Focus ST BTCC (2012)
Ford Fusion NASCAR (2012)
Ford GT40 MkIV (1967)
Ford Mustang 2+2 Fastback (1966)
Ford Mustang Boss 302 R1
Ford Mustang Cobra SCCA Trans Am (1997)
Ford Mustang GT (2015)
Ford Sierra RS500 Cosworth GroupA (1998)
Ford Zakspeed Capri Group5 (1980)
Formula 1 (SMS/Generic)
Formula 2 (SMS/Generic)
Formula 3 (SMS/Generic)
Formula Gulf FG1000 (SMS/Generic)
Formula Ford (SMS/Generic)
Ginetta G40 Junior
Ginetta G50
Ginetta G55 GT3
Gumpert Apollo Sport (2011)
Lotus 25 (1962)
Lotus 38 (1965)
Lotus 40 (1965)
Lotus 49 Cosworth (1967)
Lotus 49C Cosworth (1970)
Lotus 51 (1967)
Lotus 56 (1968)
Lotus 72D Cosworth (1972)
Lotus 78 Cosworth (1977)
Lotus 98T Renault Turbo (1986)
McLaren F1 (1994)
McLaren F1 GTR (1997)
McLaren P1 (2013)
McLaren MP4-12C (2013)
McLaren MP4-12C GT3 (2013)
Mercedes-AMG C-Class Coupe DTM (2013)
Mercedes-Benz 190E 2.5-16 Evolution2 DTM (1991)
Mercedes-Benz 300 SEL 6.8 AMG "Rote Sau" (1971)
Mercedes-Benz 300SL W194 (1952)
Mercedes-Benz A45 AMG (2013)
Mercedes-Benz CLK-LM (1998)
Mercedes-Benz SLS AMG (2011)
Mercedes-Benz SLS AMG GT3 (2012)
Mitsubishi Lancer Evolution X FQ-400 (2012)
Oreca O3 LMP2
Pagani Huayra (2012)
Pagani Zonda Cinque Roadster (2009)
Pagani Zonda R (2012)
Palmer Jaguar JPLM (2011)
Radical SR3 RS
Radical SR8 RX
Radical RXC Turbo
Renault Alpine A442B (1978)
Renault Alpine A450 (2013)
Renault Clio IV RS Cup (2013)
Renault Megane RS (2014)
Renault Laguna BTCC (1999)
Renault Mégane Trophy II (2009)
Ruf CTR "Yellowbird" (1987)
Ruf CTR3 (2012)
Ruf RGT-8 (2013)
Ruf Rt 12R (2012)
Sauber C9 Mercedes (1989)