Colored coins do exactly what you want - they don't need a new blockchain or a change to the Bitcoin protocol, they allow representing arbitrary assets and contracts with atomic trades, and already have working (though experimental) implementations for a color-aware wallet and p2p trade.

bitcoinx.org is outdated, soon http://coloredcoins.org/ will be up to provide information. In the meantime here is a promotional video: http://www.youtube.com/watch?v=fmFjmvwPGKU

I see people have mentioned Mastercoin, it's much more ambitious but is also technically inferior, and more centralized (Mastercoins were issued with a time scale factor of 1 month).