Grinder34
Track Monkey
http://www.amazon.com/Graco-4045-Seatbelt-Locking-Clip/dp/B0002ADQOU
Its a seatbelt lock. A huge problem at the track, even if you don't realize it, is sliding around in your seat, and using the steering wheel to stabilize yourself. Almost all street-legal cars don't have a harness, which you need to firmly hold you in place. Some people use harness bars as a work around, but there are some serious safety concerns with those.
This $5 piece will lock your seatbelt in place, and will keep you VERY snugly planted in your seat. The competitior to this product is the CG Lock, which does roughly the same thing for >=10x the price. I've heard the CG-Lock also abraids the seatbelts in a bad way.
It's $5 and its awesome. If you have instructors/passengers, get two...they'll thank you!
Here's how it installs (this is the CG-lock, but they both install the same.)