Valve is obviously at the top. :)

Lightbox... Hello?!?
I am also inclined to add many iOS/Android developers who give their games away for free and/or add levels to their $0.99 games all the time.

Blizzard is being pretty generous with starcraft 2, even when their releasing a new expansion they upgrade your online game for free so you can stay competitive.

ninja hunter, totally agree....starcraft 2 is the single best competitive game ever made imo, and the starcraft arcade offers 1000's of hours of free mods and maps

valve, source engine, 1000's of hours of mods, total conversions, steam, steamworks, project greenlight, amazing sales, by far, BY FAR, the most generous of any company to the community

cd project are AWESOME as well, tons of free content for the witcher 1 and 2

atari, with neverwinter nights 1 and 2, amazing mods, mmo's and free campaigns, literllay 1000's of hours of free gameplay.

crytek, with crysis 1, the mods are AMAZING, many better than the base game

epic, with the unreal engine, unlimited potential, 1000's of hours of free mods, total conversions

all of these games on pc, where, just by adding mod tools you extend your game, you give it life and you let the community run wild with it.

Valve - Obviously. Don't even need to reason this.

CD Project - For always providing enhanced editions and more contents free for existing customers, especially in an era where other developers are nickle'n'diming you for virtual goods.

Bungie - one of the only companies ever providing free DLC for console based games. Not to mention their intense support of the online community, the tools they release to the public, and the encompassing service they started providing with Halo 3 (stats, picture uploads and even film rendering on their server farms).

Bethesda - For not just releasing their editing tools to the public, but for also providing the most comprehensive mod management tools .

ID Software - The people responsible not only for the FPS as we know it, but also for the best mod community support ever since the doom wad files. Not to mention, they release their old engines as open source so people can benefit from them for free.

Where is EA, come on guys!

(I laughed while typing that)

2-5 for me ,1 flip-flops way to much .

From my xbox 2005-2012
1. id say which is hard to believe now a days but
Activision/Infinity Ward Call of Duty 2 (not big red one)
Two free maps .
2. id say Rare/Perfect Dark; which gave me 2 remakes maps
from goldeneye
3. Bungie for their support in halo 3
4.Turn 10, i think im pushing 30 cars gifted to me by the studio

Valve is my #1 for sure.