Prosper.com is a great concept and I have been watching the site with some interest for a while; I'm really curious to whether it will be able to expand into the mainstream. I think there are a lot of trust issues; but they are doing a good job managing them for now.
If you are interested in micro-lending as a charitable concept instead, check out kiva.org. It's similar to propser.com in that people join together to provide microloans, but these loans are interest free to help 3rd world aspiring entrepreneurs - its crazy looking at the things people are looking to get money for, you can help someone buy a goat to start their business
Anyway, it's kind of cool spin on charitable giving. For now, it appears that people do seem to be getting paid back, so it is working. I think most people treat their contribution as a charitable gift and tend to just reinvest their loan proceeds in other microloans, but you do have the option of taking the money back out once you have been repaid.
I know, I know, I'm way off topic here as this isn't even about making any money...