Are the keys codes or tradeable items?
Key codes, or some of them can also be sent as Steam gifts.
Which ones of them can be sent as gifts?
Er.. just kidding about the ability to send as Steam gifts, I guess. Sorta.
I can send some of them as gifts via
Humble Bundle, which removes the gifted game from my Humble Bundle library and provides the giftee with a link to a page where they claim and reveal the key code to activate the game on Steam.
I am able to send these as gifts through humble bundle:
Dragon Age Origins
Dead Space 2
Legend of Grimrock
Terraria
Sanctum 2
*edit* I have these as well:
Galactic Civilizations II: Ultimate Edition
The Incredible Adventures of Van Helsing - Complete Pack
Rocketbirds: Hardboiled Chicken
Awesomenauts
Wanderlust: Rebirth
Just to note - a game's key is never revealed/visible to me when gifting through the above method.