We're quite fond of what Roush does with the 2010 Ford Mustang GT. Check them out here, or read our review of the 427R.
For generations (of the car, of course), people have been making this hot hatch even hotter. SFX Performance is one source for GTI modifications. Autotech Sport Tuning is another.
Subaru Impreza WRX:
When someone mentions tuning, the WRX is the first car that comes to mind for many people. Cobb Tuning offers some performance upgrades, including the AccessPort, which we love.
If the Civic Si just doesn't cut it for you, Pro Civic should give you some ideas on how to mod your Civic.
Being such a popular vehicle, there are many sources for Corvette mods. Corvette Guys is one. Techco makes a scary supercharger for the current 'Vette.
Horsepower Freaks has a variety of mostly appearance upgrades (including Lambo doors) for the rotary-engine Mazda we love so much. Strike Engine has a list of other RX-8 tuning resources.
The 370Z Nismo is sublime, but if you want to do it yourself, try Zele Performance. Also try the Outperformance Shop.
Mitsubishi Lancer Evolution:
We were going to put the Ralliart on the list, but the thought of tuning out an Evo gets our hearts racing. STI Killer is a good place to start. Lancer Shop has some evil Evo goodies, too.
Dinan will help you make an already great car even greater.
The xB begs to be modified, whether in appearance or performance. SFX Performance has some parts available. Fast Scions has a lot to offer.