have multiple streams of passive income

I have an impressive collection of editorial stock photography—however, it is very niche, and needs to be actively marketed to turn into a decent stream of income.

I’d love to do more artistic, high-concept stock that’s difficult to duplicate.

...has appreciated $50,000 in two years. Not too shabby. Now if only we could afford 10 more just like it so we could take better advantage of this good market…

The trouble with real-estate, though, is that assets are not liquid. I hate being cash-poor.

In the mean-time, I’ve got several websites running adsense—none of them earning any appreciable income, yet, unfortunately, though one is at least showing signs of life…

What are some other good sources of passive income?