Micro finance lending with interest rate :
In the USA : Lendingclub.com (7.45%/y), Proper.com (8.89%/y), Fundingcircle.com (5.7%/y), Zopa.com (1.36%/y).
In Europe : Isepankur.ee (21%/y).
Some guys on Isepankur are reaching 28-30% per year (sources 1, 2).
The repayment rate has to be considered for the final net interest.
Depending on your location, the taxes on such investment can be high or the investment itself can be forbidden by the law.
Micro finance lending without interest rate :
Kiva.org, supported by Bill Clinton. You invest 25$ on different projetcs, you get back your money at the end of the loans and you can reinject it into other projects. You can join a group of people to select the projects, etc...
There are other similar services, but they use intermediaries that propose 25% of interest to the loaners... contrary to Kiva.