Here is all the popular Mustang 3rd party makes: Saleen, Steeda, Roush, SVT
Saleen sells 3 different types of Mustangs. The N/A S281, the supercharged S281 SC, and the bored, stroked, and supercharged S281E (Extreme). There is alot of newer Saleen models that I did not mention, as these 3 are the main ones.
Steeda maks mostly track-prepared Mustangs. Steeda upgrades the suspension on the Mustang drastically. They are not into power that mush, as they are in handling.
Roush does not just make body kits. Roush was founded by Jack Roush
, a retired WWII P-51 Pilot. Roush Mustangs are similar to the Saleen lineup, but they have a Stage 1, Stage 2, and Stage 3 model instead. The Stage 1 and Stage 2 are both naturally aspirated, but come with the body kit, and a few bolt ons. The Stage 3 model has a Roushcharger supercharger equipped.
SVT, sadly, is no longer around, although they still make the motors for the GT500's. SVT was founded in 1993, in which that year they released the 5.0L Mustang Cobra. From 1993-2001 they made the Mustang Cobras, the most popular Mustangs to date. Int he years 1993, 1995, and 2000, they made the Cobra R model. The R model was naturally aspirated, but they were built for track. The 1999-2001 Cobra's got a complete enmgine swap. SVT dumped the stock 24V SOHC 281 and dropped in a 32V DOHC 281, making 320 hp. In 2003, the Cobra returned, with the same motor, but with a 2.3L Eaton supercharger. They also changed the bodykit and rims.
Shelby is the most historical of the brands. Founded by Carol Shelby in the '60's, Shelby has made some of the most powerful Mustangs since they have been around. Their latest Mustang, the 2010 Shelby GT500
, makes 540 HP with a 5.4L Supercharged V8.
I hope this helps, and if you have anymore questions just ask.