Believe it or not, Alan Wake is on GOG! The reason I'm so surprised is that it was first released in January 2012, which easily makes it the most recently commercially sold game in GOG's collection.
From what people have been saying on Steam, the only difference between the GOG and Steam versions (other than DRM) is that Steam's includes a soundtrack. Personally, I don't give a damn about that and considering Alan Wake is 50% off -- on both GOG and Steam -- now seems like a good time to buy it.
Steam has two excellent, today-only specials for fans of iD Software games! The first is the Wolf Pack, which contains Wolfenstien 3D, its prequel, Spear of Destiny, and Return to Castle Wolfenstien, for $3.75. The second is the Super iD Software Pack, which contains the complete Quake, Hexen, and Commander Keen series in addition to RAGE and the contents of the Wolf Pack, all for $49.99.
You like manuals. You like manuals. You love them. You cannot resist manuals. Your gameplay experience is meaningless without manuals.
Steam is having it's annual Summer Sale right now. In addition to the regular sales, there are "flash sales" (short-term sales), vote sales (where you vote on which games goes 75% off) and daily sales! It ends on July 23, so check it out!
So far, this Steam summer sale has been the best haul I've ever taken in. I literally have enough games to last me the next couple years. Alan Wake, Arkham City, Braid, Costume Quest, Dear Esthar, LA Noire, Limbo, Mirror's Edge, KOTOR, Syberia, World of Goo and the complete Grand Theft Auto series. I'm even getting a PC copy of "Psychonauts" for $2.50 (which I recommend EVERYONE does.)
I'm still holding out for the end of the day in case they do anything with "Half-Life 2" or "Mass Effect". Those flash sales have been absolutely fabulous.