It's great living in NJ/NY.
East Coast Supercharging (does a lot more than just blowers. A lot of my close friends prefer ECS. They're expensive, but they build a shit ton of reliable power.)
http://www.eastcoastsupercharging.com/
Cartek
http://www.cartek.net/
TT Performance (TTP. Some of my other friends swear by TTP. They've always been treated well.)
http://www.ttperformance.net/
Tune Time Performance (not TTP, but I hear that name a lot with the TBSS guys)
http://www.tunetyme.net
Kaltech (Kaltech sponsors a close friend of mine, and her GTO runs like a raped ape consistently enough to be competitive)
http://www.kaltechtuning.com/
Corvettes of Westchester (did the install on my friend's heads/cam car)
http://www.corvettes-of-westchester.com/
Vette Doctors
http://www.thevettedoctors.com/
and so on and so forth.
Then you have your big names, all over the country.
Katech (they do the motors for the C6.Rs and build some of the baddest, most durable road race LSx cars. Hope you have a lot of coin though.)
http://www.katechengines.com/katech_inc/
Lingenfelter (big coin, I've heard questionable things since John died. Still a big national name in tuning though)
http://www.lingenfelter.com/
LG Motorsports (decent coin, very established in road racing. Nasty cams, and a big selection of them. Very popular.)
http://www.lgmotorsports.com/catalog/index.php
Texas Speed (their cam grinds are very popular. See their cams in a lot of cars.)
http://www.texas-speed.com/
Quality Motorsports (don't know much about them besides the fact that they hit 600whp with their heads/cam in the LS7, so they're popular with the C6Z crowd)
http://www.quality-motorsports.com/
That's all I can think of off the top of my head. I know I left out a lot, but that's a start.