Kiva.org is a popular microloan organization. It is essentially a very successful non-profit advertising agency for a multitude of local non-governmental and not for profit organizations around the world, allowing computer literate Westerners to easily fund microloans around the world. While making loans to developing countries always carries some risk, Kiva has a very high repayment rate, currently just over 99%.
Kiva lists loan opportunities from 194 microloan organizations in 68 different countries, in categories ranging from agriculture to art. Those seeking loans are easily searchable by loan type, country, sex, group/individual, and a number of other attributes, including 'youth', 'vulnerable groups', 'conflict zones', 'fair trade', and many others. Those making loans may do so in increments of $25, to be held by Kiva until the requested amount is reached; the site makes it very easy to see how much has been raised towards any particular loan, and once the amount is reached, how much has been repaid.
Kiva's main draw is that it makes loaning easy and fun. You can easily see what you have loaned, when you will get it back to re-loan it, and how much you have loaned overall. You can post your loans to your favorite social media sites, you can join loaning teams (of which there are thousands), and you can follow various group discussions. Paypal will waive their usual transaction fees for transactions to and from Kiva, and a small donation to Kiva to help maintain the website and research lending partners is optional. (This means that if you decide to withdraw repaid loans rather than relend, your stint in charity need not cost you anything). Kiva accepts the major credit cards.
Kiva is by far the most popular site of this sort, in large part because it has been quite successful in using social media, particularly Facebook, to advertise. They have had a lot of success with their 'trial loan' program, in which a new user is granted $25 from the Kiva slush fund to loan; obviously, the new user does not get the $25 back after the loan is repaid, but it has still been extremely successful in getting new users to join. Another major hook is the sale of Kiva gift cards, which are quite popular around the holiday season; they are available in increments of $25, and if the receiver so chooses they can be withdrawn (as Paypal credit) after the term of the loan is up.
Kiva is very popular in Europe as well as America, and has local teams in most countries (admittedly, some are very small teams; the Turkish team has only 125 members). There are some rather large groups that dominate the lending teams, some of which are rather partisan; currently the 'Atheists, Agnostics, Skeptics, Freethinkers, Secular Humanists and the Non-Religious' team leads the all-time loaning record, with over $11,000,000 lent, followed by the 'Kiva Christians' with over $7,000,000. The Everything2 lending group has given a grand total of $1,075.
While Kiva has been well inspected by those groups that inspect charitable organizations and always recieves good reviews, it is worth noting that microloan organization have recently been decried as being less effective in causing long-term change than those organizations that provide medical support and other forms of basic aid. If you are looking for the most bang for your buck, I would recommend you look at the GiveWell blog.
Charity Navigator's Kiva page (5 star rating).