GM-era forced-induction anything.

1. Any Saab. 4-cylinder turbo 9-3 Aero tops the list. The V6 Aero is a marvel, but with slightly more frontal weight bias, as well as being more refined in power output, thus not as fun. Prices are insanely low for what you get.

2. Chevy HHR SS. Not plagued with the problems of the non-SS models, but absorbs their price hit. The 2.0 turbo direct-injection motor (the bane of modern turbo motors existence, I know) is stout and tuneable. These can be had for $6-7k. Good luck in the search for the 2009-only panel model.

3. Saturn Ion Redline. First gen for a supercharged model, later years for the aforementioned 2.0 DI Turbo. Right now running at $5k for models with a smidge over 100k miles.