Steam is currently running a beta program that allows you to collect and trade cards that are found through playing various steam games.
join the group to join the beta
http://steamcommunity.com/groups/tradingcardshowever a more pressing matter is at hand and that is that I have some cards I'm looking to trade.
FormatSteam Profile ID: [SteamID]
Cards I want: [Game]-{Card Name}
Cards for trade: [Game]-{Card Name}(Cards Available)
Steam Profile ID: MrRazot
Cards I want: HL2-Respite, HL2-Touble Underground, Portal2-Any
Cards for trade: HL2-City 17 Metrocop(2), HL2-Gordon Freeman(1)