I'm the founder of Bitcoinica. It has been featured by Hacker News, @bitcoinmedia, etc.

I recommend you to give Bitcoinica a try, it's really advanced!

https://www.bitcoinica.com

(You should visit the website for a real feeling. The texts below are copied from homepage.)

Elegant.
Professional.
Revolutionary.
Margin Trading, Stop Orders, Guaranteed Liquidity, Short Selling, and many more.


Why Bitcoinica?

Advanced Trading

We're the first Bitcoin trading platform that supports Market, Limit, Stop, Trailing Stop order types.
All accounts are by default Margin Accounts, which means that you can trade more than what you deposit.

Security

We don't operate a Bitcoin wallet. All our funds are stored in Mt. Gox and our bank accounts.
Bitcoinica runs on Heroku, a secure cloud platform that hosts thousands of popular websites.

Forex-like Experience

Unlike other Bitcoin exchanges, we guarantee liquidity. All exchange rates you see are meant for any orders under ฿50. No transaction fees!