Although it's sometimes a bit of work to find these opportunities, there are enough people with good ideas who need money these days. Friend of a friend-type stuff.

Occasionally, after talking to a startup about a potential employment opportunity, I'll ask if I can simply invest. They are usually flattered that someone would be so excited, and they are usually quite happy to take your money.

A final option is to invest in one of these new index funds that track startup performance. E.g., the SharesPost 100.