Impulse::Reactor uses GOO as DRM, but requires no client to operate. Games utilizing Reactor wouldn't even need to be bundled with Impulse, Stardock's digital distribution platform. Furthermore, these games could be sold at retail or even on other digital distribution platforms like Steam, Direct2Drive, etc.
The tools are free to use and the only requirement is that any game utilizing the framework also be sold on Stardock's Impulse service, though not exclusively. This is in contrast to Valve's Steam exclusivity requirement to use Steamworks for features and DRM. (Update: Games that use Steamworks can be sold through any outlet, though they require a mandatory Steam installation. Steamworks features can also be used alongside other services, such as Games for Windows Live, as seen in Dawn of War II.)
That's great, but what does Impulse::Reactor get you? Well, how about GOO for DRM, friends lists, Facebook and Twitter integration, in-game DLC, achievements, chat, matchmaking, rankings and ladders, server lists, and cloud storage for game saves.
Finally, the overlays for these features can be skinned by the developer to match the look of the game. It remains to be seen if developers will flock to this framework for features and DRM. On the other side of the coin, Valve and Epic just announced a partnership to include Steamworks in Unreal Engine 3, which would make it easier for developers to use Steamworks in UE3 games.
More information on Impulse::Reactor can be found on Stardock CEO Brad Wardell's blog.