As many of you know, Humble Bundle has moved away from codes completely. Now days you have to link your Steam account to Humble Bundle and then the games get directly added to your library. You can still give/trade the games from humble bundle but then the buyer has to "redeem" the game and what nat...the receiver/buyer basically gets a gift link which takes him/her to humble bundle...and so on... I have written a step by step guide on how to use the gift links, the guide can be found here.
While it does add some complexity, I actually welcome this change. Since the addition of this feature, Humble Bundle seems to actively support giving away your spare keys. Before that it was somewhat of a gray area.
"some complexity" is an understatement. I think it's way too complicated (or at least for those who sell these games), the gift link system can be extremely confusing from time to time and I also bet that soon there will be some phishing shit based on this gift link system. Even tho if people would just read and use their head for a bit more than just to carry their hats and hair on then no phishing links or attempts would ever be successful.