Three Thrifty Guy's list of The Fastest Sport Cars For Sale Today Under 20k. Feel free to contribute, you must be able to buy the car for less then 20k.
http://www.ranker.com/list/fastest-used-sport-cars-under-20k/steveeckles,
BMW M3
2001 BMW M3 Coupe 0-60 mph 4.6 Quarter mile 13.2
1998-2004 sale for 7-18k
Chevrolet Corvette
2005 Chevrolet Corvette C6 0-60 mph 4.2 Quarter mile 12.6
2005 is the new model (C6) sale for 19-25k BASE
1997-2004 model (C5) sale for 10-25k (base < Z06)
Honda S2000
2000 Honda S2000 0-60 mph 5.7 Quarter mile 14.0
Mitsubishi Lancer Evolution
Subaru Impreza WRX STI
2003 Subaru Impreza WRX STI 0-60 mph 4.7 Quarter mile 13.1
1993 Mazda RX-7
Nissan 350Z
2002 Nissan 350Z 0-60 mph 5.1 Quarter mile 13.5
mercedes clk 55 amg
2001 Mercedes CLK55 AMG 0-60 mph 4.9 Quarter mile 13.4
porsche 911 Carrera
1999 Porsche 911 Carrera 4 0-60 mph 4.9 Quarter mile 13.2
Ford Mustang SVT Cobra
2004 Ford Mustang Cobra SVT 0-60 mph 4.4 Quarter mile 12.6