The stock drive shaft is a 2-piece unit which weighs in at approximatly 18 lbs. The front section is transmission specific (auto or manual) and the rear section is ABS specific (either you have it or you dont). There is a carrier bearing in the center of the 2 pieces which can be used as a catch if you were to upgrade to a aftermarket one piece drive shaft.
Aluminum Drive ShaftEdit
Many places sell them for arround $400. The are substantially lighter and stiffer helping you transfer more power to the wheels.
Carbon Fiber Drive ShaftEdit
These are not very much lighter than the aluminum version, but they are safer. If you drop the aluminum one it will break and then rotate around destroying the bottom side of your chassis and possibly making you crash. The carbon fiber one will disintegrate as soon as it hits the pavement, not harming anything besides itself. IMO is not worth the extra money, just be sure to use have a drive shaft catch and check the bolts before and after every track day.
A 1 piece driveshaft can be installed in lieu of the stock 2 piece. This will by nature reduce weight and decrease drivetrain "slop" over stock. There are various companies that produce these in steel, aluminum, and carbon fiber based tubes. You may also bring your current one to a local driveshaft repair facility to recondition the stock U joints and convert it to a single piece tubular steel driveshaft for low cost. Note that driveshafts are all different based on whether or not its an s13 or s14/auto or manual/and if equipped with abs or not.