[GeeTwo]

Building materials (aluminum stock and hardware, esp versaframe & gussets & rivets & nuts/bolts)
Wire, tubing, and other basic electric and pneumatic components.

Wheels: We don't really buy wheels just to stockpile, but we never throw away a wheel that still has enough life left in it to serve as a prototype.
Controls stuff: again, we don't usually buy anything specifically to stockpile, but everything that doesn't work out from prototyping that's still useful goes on the shelf.
Actuators: We have motors and cylinders that are not up to another life on a competition robot, but we keep anything reasonably serviceable to use in prototypes.
Sensors: We keep a bunch of odds and ends limit switches, range detectors, and encoders for the gearboxes we like to use.
Chassis Kits: This year, we opted out of the KoP, but bought two AM-14U2s a few months before the season. This let us brainstorm ideas with a known kit before kickoff. It looks like it paid off - we've done much more with the kit than the designers ever intended. (We still have to prove that it works, though.)

Do not stock up on FRC-specific items unless you are reasonably confident that you'll use them, or you have enough money to put at risk. Most items are eventually de-listed from being FRC legal; you don't want to spend a bunch of money on motors and control items that you can't use. I hope that FRC continues the policy they started this year of announcing legal motor controllers and motors in the fall, and extends it to more items and longer lead times.