Any game you’ve purchased from the store as a gift, or received as an extra copy, can be traded to other users. They can be used to trade for other gifts too, or for items in Team Fortress 2.
"We’ve added a new checkout option to the Store when purchasing a gift so you can save it for trading or sending later, to support users who want to save games for trade fodder," says Valve.
Valve have also said that it's working closely with developers to incorporate more games and items in the coming months.
To gain access to the beta, head to your Steam client, click Steam, Settings and select the 'Steam Trading Beta' option.
For more information check out the Steam Trading Beta FAQ.