http://www.mmobomb.com/giveaway/warframe-beta-keys (9000 more posted)
http://www.freemmostation.com/giveaways/warframe-closed-beta-giveaway/ (7000 more posted)
I just started playing this last night with some friends, it's super early in beta but it's already really fun gameplay wise, if a bit repetitive map/tileset wise because it's so early.
The basic premise is it's 4 player class based co-op 3rd person robot space ninjas. You take on various mission types like Sabotage, Defense, Spy, Assassinate, Exterminate, with seemingly randomized objectives, so playing the same mission twice could be different.
As you play you level up and earn points to put in a skill tree, which reminds me of how the weapon upgrades work in Dead Space. As far as character upgrades there are Mods to put into Mod Slots you can pick up, they give you things like +armor, +shields +fire damage, etc. There are also crafting materials and blueprints to create new weapons or warframes. Weapons level independently of the warframe you're using them with, and in game there are bonus objectives (get 10 headshots, etc.) to gain bonus affinity for your weapons. Weapons use the same kind of skill trees that warframes do, and can also take mods.
It's a free to play game, so there are some microtransaction bits, the grossest of which is you can only revive yourself 4 times per day without buying more, other players reviving you is unlimited though and doesn't use one of those 4. Most of it is cosmetic or early unlocks though, and there's no player vs player, so it's not that big of a deal really.
If you need more convincing, or want to see it for yourself, here is a video.