For awhile Bitcoinica was a very popular leveraged trading service, but it got hacked and closed down. Are there any more services like that still functioning in the Bitcoin ecosystem?
MPEx operates MPOE for options trading. Though options are a financial derivative in what a smaller amount of funds is used to speculate on the price move in the valuation of a much larger amount. For example, BTC/USD options contracts costing 0.1 BTC could triple in value should the BTC/USD exchange rate move by a couple dollars, for instance. MPOE's options exercised are settled in cash (bitcoins). However, MPEx does offer margined trading (essentially a loan that can be used for trading).
ICBIT.se offers leveraged futures contract trading. It offers futures contracts for the BTC/USD exchange rate, for Gold and for Crude Oil (WTI). These are cash-settlement (in bitcoins). Leverage can be up to 10X.
There are various Forex shops which accept bitcoins as a method for adding funds and which also offer leverage.
https://icbit.se/ offers trading of futures instruments, in particular BTCUSD-12.12 (BUZ2) which enables leveraged BTC trading. However, user orders are matched only against other users (it does not utilize liquidity from larger exchanges such as Mt. Gox), and the way it works is not very intuitive.
BitFinex was built using the leaked Bitcoinica source code.
http://xcfd.com — gives bitcoin CFD with 1:5 leverage.
You can find some brokers which allow leveraged BTC/USD trading here.
Make sure that you're aware of the risks involved in such investment as bitcoin value became extremely volatile lately and the leverage makes it even more dangerous.
Also don't forget to examine the broker's security system. As mentioned above, bitcoin marketplaces are hackers favorite target nowadays.