Last month, Valve made a rather generous offer to genuine developers of the Linux-derived Debian operating system: drop us a line and we'll give you the entire library of Valve games free. Unsurprisingly, such a juicy offer has had its share of scam attempts, though you can rest assured that the offer is being well policed.
According to Collabora's Jo Shields, who is responsible for verifying the claims, the majority of responses have been from legitimate devs, with 279 approved so far and only 22 enquires refused. Of those 22, 10 were honest requests that didn't meet the criteria, while seven were half-hearted (and easily identifiable) scam attempts. The last five, however, can only be described as incredibly dodgy.
As Shields explains, the requests try to look valid, but make elementary mistakes -- mostly involving not signing their emails correctly with GPG. Even if this system wasn't in place, I'd probably raise an eyebrow at any email that contained little more than the sentence "I'm would like to get the valve produced games".
So, if you were thinking of trying one on in the hopes of bagging yourself all of Valve's games, you're extremely unlikely to succeed.