Second Life has its own currency, Lindens. On any given day people are using it to conduct transactions. They buy items in world (clothes, toys, etc.) and they buy items out of world. People can cash Lindens out of the game on the Lindex (you need an SL id to see this page, but the current going rate is 314L$ to the $1) or use it to buy new ones.
The Lindex has been around for a while, but Linden Labs (the creators of Second Life) only recently added their Economic Statistics page. (It originally went up open to the world, now it’s restricted to SL members only, but here’s a link to the Google cache.) This page tracks Land & Island Sales and Resident Transactions (all in Lindens). They had a total of 5,266,511 transactions in the month of April. That’s a lot for a world with 200k users; what are they all for?
Looking at the volume of transactions in Lindens, I decided to convert them to dollars. I used an exchange rate of 300$L to the US$. This is less than Lindens are currently trading for (314$L), but a good average for the entire month of April.
The amazing thing about these transactions is that over 85% (just under 4.5 million) of them are conducted for amounts under a dollar; 57% of them are conducted in amounts under $0.07. Transaction amounts like that are not cost effective when you are dealing with credit cards and perhaps only slightly better when dealing with Paypal (however they still charge a $0.05 fee per transaction).
I wonder if people will start using Lindens more out of world than in. It currently costs nothing to get an SL ID and you never even have to enter the world to begin collecting Lindens.