is a GPG based exchange for BTC stocks. Users sign orders, encrypt them with the exchange's public key and send them over (as a POST["msg"] to that url or else paste it in the form on the page). The response is signed by the exchange and encrypted with the user's public key.
You can STAT, BUY and SELL so far. Depositing and withdrawing, order cancellation, listings and graphs to come. Hopefully if no major bugs are found MPEx could go live sometime this month.
There's a 20 BTC fee to have your public key loaded on the exchange keyring, but for as long as the beta lasts that fee is waived, so take advantage. You also get some paper to play with. This offer is only open for active forum users or people with an -otc WOT.
Now go forth and break the thing.