Why am I missing all these free game promotions from other websites?

Seems like a lot of people received loads of free games. Hacker Evolution, Shadowgrounds and all this. Seems like amazon and GMG give those free promotion offers sometimes.
My question is - is there like a website or a steam group or something like that which notifies what kind of promotions and free games are happening around the Internet? I seem to be really slow to notice and join all of the promotions and giveaways so it is kind of a bother. I mean, I HAVE managed to get free games like Portal and Half-Life 2 Deathmatch, but that was because someone told me there was a promotion. Yes, a stupid and beefy question indeed, but nevertheless, I am curious (if you are willing to share it of course, considering free games have a quantity cap, such groups would be hiding under a rock or something).