Springs
-Great for plushness.  It takes very little to activate it over the first
bit, which is great for very small high frequency bumps.  It also doesn't
"stack" as easily.
-This is the best upgrade for forks with some sort of dampening units.
Springs are easy to compress, however, it reacts like a pogo stick.  If
the fork is not dampened, the fork might become very bouncy.

Hydraulic Cartridges
-There are 2 kinds.
a)If the fork is a normal elastomer based fork, it might be possible to
retrofit the unit with a hydraulic unit on one side to give the fork some
dampening.
b)Some of the newer forks have problem with cartridge leakage.  The
aftermarket cartriges are supposed to be stronger and more leak
resistance.

Air Cartridges
-Very plush and provides great dampening.  
-Hard to bottom out
-Can be a pain to maintain due to leakage.  Some users find that they have
to pump the cartridges very often.

Brake Brace
-If the fork is too flexy, this will increase the stiffness.
-Brake bosses will flex less, which is ideal for hydraulic and linear pull
brake users.

For other fork upgrades that do not involve buying, but rather drilling
and fiddling, check http://www.magi.com/~kroberge/kmrtop.html and see his
tech page.