OK, I've just done this myself, so if it does turn out to be in any way dodgy at least you can rest assured that I'll be in the same trouble as anyone else who takes a punt on it. So far as I can tell though, it's simply a promotional stunt for a German download portal called DLH.NET. To anyone who signs up at the moment, they're gifting free Steam codes for olden hacker-slasher Enclave, 'strategic economy simulation' Knights & Merchants HD and naval strategy game East India Company Gold. None of them are humdingers, perhaps, but hey, they're going for no-pennies if you want 'em.
Fascinatingly, East India Company Gold isn't even available to buy on Steam, but I can attest to the fact it can be activated on it. What you have to do is head here, click on the German flag top-right and select the English one instead (unless of course you speak German fluently), sign up, wait for the activation email, click the link in that, login, go to your profile page and then you'll see the three games listed, with links to activate them. It all worked for me, and as far as I can tell my identity hasn't been stolen yet. I was able to opt out of the newsletter when I signed up, and so far there hasn't been a torrent of crap into my inbox.
The only real wrinkle is that wou will need to repeatedly switch the language on DLH to English, as it defaults back to German repeatedly. Also some of the sign-up emails are in German, but it was pretty obvious what I needed to do. Frustrating, but there's no such thing as a free lunch. There is such a thing as lunch that you only have to pay for by changing a website's language settings multiple times, however.