There is a page that everyone in Second Life should know about. It is the Second Life Economic Statistics page. If you're an economist, this is an astoundingly fun page just to wonder at the existence of such a virtual economy.
This page includes stats on population, land and land sales, Linden $ supply, number of in-world business owners with positive L$ cash flow, monthly spend by resident, and sources and sinks of L$.
The main reason most everyone needs to know about this page is average L$ paid per sq meter or land. While the old adage is true "location, location, location", it is often useful to see what the price you may pay for a piece of land looks like as compared to the average. Is your land not selling? Maybe you are asking to much.
What's probably one of the most interesting statistics is the estimated number of residents making > $5000 USD. Starting in April 2007 this number has not moved a whole lot relative to the explosion in population and even the upward trend of unique users who have positive cash flow.
Even though the number of individuals at this range are small, if these estimates are right, than <200 people are making between $645,000-$725,000USD a month on this virtual economy.
Pretty impressive for these early days.