Second Life is a virtual world with many million participants created by Linden Labs. Your digital representation in this world is called an avatar. You can make your avatar wander around, buy houses, design clothes, get married, whatever you like. Think of it as World of Warcraft without the weapons. There are a few interesting features of Second Life that make it interesting from a security perspective. The currency used in the virtual world, Linden dollars, can be exchanged for real currency in a fully supported way. The exchange rate is something on the order of 270 L$ for 1 USD$. Therefore, if you could exploit another player's client, you could potentially cash out their Linden dollars for real money.