Here's what crates can give you (or take you):
Armor - The unit that gathered the crate will have upgraded armor with a (initial armor x1.5) value.
Cash - $ added to your initial $.
Firepower - Double Damage (initial damage x2).
Promotion - Units near crate will be promoted to Elite.
SpySat - Shows you all the map (except under GAP/Stealth).
GAPGen - Anti-SpySat, will GAP all the Map. To counter: sell your SpySat/Adv.Radar/C-Ball and rebuild it.
Speed - Upgrades speed to 1.2 of initial value.
Resource - The crate will spread some resources near.
Explosion - The crate will blow up destroying most of the units (few can resist the blow).
Napalm - Same as above, but explodes with napalm.
Gas - The crate will generate a lot of toxic gas after explosion.
Free4U - You get a free unit. It can be anything from Miner to Ship.
Chrono - The crate will chronoshift to another location when you try to approach it.