There are a LOT of crates in the game, and finding them and getting to know the different effects is part of the fun in my opinion. ;)
If you want an idea of the things to expect though, this thread from the Introversion forums attempts to list them and give details of their effects.
EDIT: Apparently it's not recommended to just link answers, in case the site disappears and so forth. So I've copied the info from that thread and split it into sections of vaguely related things. Credit to "xyzyxx" on the IV forums for the original post. Possible spoilers follow - you have been warned.
Area Effect (these effect an area around the crate, or where deployed by a player)
Ant Nest - Allows you to place an Ant Nest on the map. Ants come out and attack anyone that comes near, including the team that created them. Ant Nest lets you place the nest yourself, while Ant Spawn will instantly spawn the nest at the location the crate was opened.
Ant Nest Spawn - An Ant Nest will instantly spawn at the location the crate was opened.
Dark Forest - Allows you to place a Dark Forest on the map. Dark Forests will consume the souls of nearby Multiwinians. It can also corrupt nearby Magical Forests or normal trees and turn them into more Dark Forest. They can also be set on fire.
Dark Forest Spawn - A Dark Forest will immediately grow at the location the crate was opened.
Magical Forest - A Magical Forest will grow at the location the crate was opened. Magical forests produce Souls that can be collected by Engineers, Harvesters, and Virii. They can also be set on fire.
Plague Outbreak - This crate instantly infects all nearby Multiwinians with Plague. Infected Multiwinians are identified by a green glow. Plague will spread to any Multiwinians the infected ones come into contact with. Infected Multiwinians will ignore your orders much of the time, they may throw grenades randomly (or rockets if they are Rocketwinians) and they will die a short while after being infected.
Randomiser - This creates a small area effect for a short time at the location the crate was opened. Their team colors will be randomized, i.e. some of them will change to the colors of your opponents' teams.
Reinforcements - 100 Multiwinians will spring forth from the crate as it opens. Their color will be the same as the team that opened the crate.
Triffids - Triffids will appear immediately at the location the crate was opened. Triffids spew Eggs that hatch viral units. These units will attack all teams.
Events (effects that are not localised to an area or team, or are otherwise Significant Things)
Crate Mania - About eight crates will fall from the sky all at once. This crate activates immediately.
Futurewinians - A spaceship descends from the sky and uses an abduction beam to capture some Multiwinians It does not discern between teams. The ship will then place Futurewinians on the map. They have the power to fire Subversion rays at their opponents. The Futurewinians become an additional team that can earn points in the game and potentially become the winner.
Infection Outbreak - The crate will release black Spam Rays as it opens. These Spam Rays can transform nearby Multiwinians into Black Virii, Black Spiders, and Evilwinians. Evilwinians, like Futurewinians, are a team that can earn points for itself and potentially win the game. Evilwinians and Black monsters are immune to Dark Forests, and are ignored by Soul Destroyers and Ants.
Mega Rage - This crate is the same as the Rage powerup, except that it takes effect instantly and affects all Multiwinians on the map, not just yours.
Slow Down - This crate takes effect instantly. Everything, including the game clock itself, slows down for a short time.
Spawn Mania - This crate activates immediately. All occupied spawn points on the map will spawn continuously for a few seconds. This will not affect teams that have already reached their population cap.
Speed Up - This crate takes effect instantly. Everything, including the game clock itself, moves faster for a short time.
Power Ups (effects some or all of a player's units)
Hot Feet - Allows you to cast a Hot Feet spell on a group of your Multiwinians. When Hot Feet is activated, the Multiwinians will move at double their normal speed.
Personal Shield - Allows you to cast a Personal Shield spell on a group of your Multiwinians. Personal Shield protects your Multiwinians from damage for a limited time.
Rage - Allows you to cast a Rage spell on a group of your Multiwinians. This allows the Multiwinians to fire their lasers continuously for a limited time.
Rocketwinians - This crate takes effect instantly and affects all Multiwinians on your team. The Multiwinians' Grenade attack will be replaced by a Rocket attack for the remainder of the game.
Subversion - Allows you to cast a Subversion spell on a group of your Mulwiwinians. This gives them Subversion Rays which can convert enemy Multiwinians to your color. If Multiwinians with Subversion active are manning a Turret, the turret will fire Subversion rays instead of bullets.
Strikes (target designated by a player)
Air Strike - Allows you to launch an Air Strike attack upon an area of the map. Air Strikes affect a small area and are capable of damaging and killing all types of units.
Eggs - Allows you to place some Eggs on the map. The creatures from these eggs will be affiliated with the team that creates them and will only attack opposing teams. Creatures may include Virii, Spiders, Centipedes, Spore Generators, and more Eggs. If one of the Eggs contains more Eggs, a Soul Destroyer may hatch from these second-level Eggs. Soul Destroyers do not affiliate themselves and will attack all teams.
Meteor Shower - Allows you to launch a Meteor attack upon an area of the map. Meteor attacks affect a wide area, and are capable of destroying all types of units. It also slows down the game for a bit, similar to Slow Down.
Napalm - Allows you to launch a Napalm Strike upon an area of the map. This works the same as an Air Strike, except that it sets fire to things rather than destroying them explosively.
Nuke - Allows you to launch a Nuclear Attack upon an area of the map. Nukes affect a medium sized area and can damage and destroy all types of units. It takes some time for the nukes to reach their designated target.
Turrets (placed by a player)
Allows you to place a turret on the map. Turrets shoot at enemies, but may harm your own units if they get in the way. Turrets are manned by multiwinians and can be hijacked by other teams if they are overwhelmed. Turrets you control can be controlled manually (
left click to select,
right click to fire,
space-bar to deselect). Turrets come in three varieties:
- Flame Turret - Shoots flame, which has a short range but is very
powerful. Also sets things on fire. :)
- Gun Turret - Shoots high powered bullets. Longer range.
- Rocket Turret - Shoots explosive rockets. High damage, but beware friendly fire.
Units (created and commanded by a player)
Armour - Allows you to create Armour. Armour will let you transport up to 100 Multiwinians across any terrain and unload them at a specific point. Armour can be terminated with the
C key, whereupon it will explode killing all Multiwinians inside and nearby.
Engineer - Allows you to create an Engineer. It will collect fallen souls and carry them to one of your nearby Spawn Points, creating more Multiwinians for your army. An Engineer can carry up to 25 souls at a time.
Harvester - Allows you to create a Harvester. Harvesters can suck up fallen souls quickly and carry them to one of your nearby Spawn Points, creating more Multiwinians for your army. A Harvester can carry up to 100 souls at a time.
Squad - Allows you to create a Squad. Squads are directly controllable (
left click to move,
right click to fire,
both click to fire grenades,
space-bar to deselect). A squad dies automatically 60 seconds after being created.