There sort of is: Profounder. More info here: http://techcrunch.com/2010/11/30/profounder-launches-to-help... and interesting thread of whether this is actually legal here: http://www.quora.com/Is-ProFounder-in-violation-of-any-secur...
looks more like a microfinance operation like prosper or lendingclub than actual investment. profits above and beyond repayment go to a nonprofit.
There are two ways business can raise money on ProFounder, either through a Private Raise or a Public Raise. Private Raises do involve entrepreneurs offering securities (based around sharing a % of revenues), while Public Raises involve sharing revenues as well, but with the actual financial gain (above and beyond the original investment amount) going to a nonprofit. So, yes, investors can make "actual investments" on the site! More here:
http://www.profounder.com/about/how_it_works