New or Used? If you're talking bout new...you don't really have much options. For $21k, I would recommend a Honda Civic SI. If you're talking about used...I would get a Subaru STI. My friend is selling one for a good price if you're interested.
For $23,000, a bit over budget: 2011 Mazda MX-5, the Miata. Starting at $20,100, the 2011 Mini Cooper:
A sports car for 15K to 20K?? I don't think those exist, dude. At least not new. Your best hope is that some of these car companies come out with garbage like the Toyota Paseo or Nissan 200SX (a glorified souped up Sentra).
Get an old car. Like someone said, a Subaru with a couple of years on it. Honestly with these smaller cars, it's not about speed, it's about getting it in a manual. That's where the fun is found.
Seriously speaking... I don't know of a "good" sports car bellow 20K. I saw your nick on the "Last Post" listing and I thought: "This mother *#(#ker is going to recommend him a lawnmower... " jk :grin: LOL!
New: Base models of Mustang, Camaro, Genesis Coupe (22k+) Used: Subaru WRX, Mazdaspeed 3, Nisssan 350z, VW GTI. I picked up a '07 Z4 with 26k miles in January for under 21k before financing. And yes, do yourself a favor and get a manual.
Get an S2000 You should get a used Honda S2000. They have hand built engines, RWD, only come in manual, Honda reliability, and all for 12-17k depending upon condition. They've also ceased production of the S2k and so they should hold their value nicely in the years to come.
I would have to suggest the Scag Cheetah. In high range it is amazingly sporty. <object width="480" height="390"><param name="movie" value="http://www.youtube.com/v/8HsgbwrWqBM?version=3&hl=en_US"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/8HsgbwrWqBM?version=3&hl=en_US" type="application/x-shockwave-flash" width="480" height="390" allowscriptaccess="always" allowfullscreen="true"></embed></object>
I don't think there's a such thing as a 15k sports car and since when was a Mazda 3 or 6 considered a sports car? lol For your price range I'd get a used Nissan 350Z or G35 coupe and call it good.