It's a virtual reality game that can be played at many pricing points, including free. Although there is a huge registered user base, there are way fewer regular users. Land and goods can be traded. The in-game currency is fully convertible thanks to a couple of exchanges that have set up. Quite a few companies are using it to research interaction in virtual communities: Dell is in there, as are Reuters, Toyota, Nissan, IBM and many others. Some of the experiments include quite interesting indeas: if you buy clothes for your avatar in-game, one shop gives you a discount on the same product in real life.
It's a good sandbox for experimenting if you're the sort of company that thinks people will spend a lot of time in virtual environments.
Some happenings in there are quite interesting: when the French Right Wing set up an office there, protesters started lobbing exploding pink pigs at them.