Some exchanges pay interest on bitcoins held in their accounts (Bter had been paying, but at all costs stay away from them), other exchanges (like Bitfinex) allow to safely loan your bitcoins to other users on market terms. But the interest is usually small, and income thus earned will be tiny unless you have thousands of bitcoins. Besides that, holding your money at exchanges is rightfully considered risky by itself. Other than that, you could find someone who would manage your coins for you, giving away a part of profits earned this way and sharing losses potentially incurred...

But this also involves risks associated with trust and managing skills of the trustee