Second Life reports (thx 3pointD) that NOAA and the Aimee Weber Studio are building a 3D visualization of live national weather for Second Life.
The system works by way of dozens of scripted reporting stations dotted all over a map of the United States. These stations retrieve METAR data from NOAA every eight minutes which they then decode and render into models of the appropriate weather phenomenon for the area. All sorts of cloud cover and precipitation models are available as well as special weather conditions such as thunderstorms and tornadoes. Temperature is represented by warmer and cooler shades of color. This 3D composite is great for giving visitors a visceral feel for the weather around them.
Anecdotally, while I was working on the partially completed map, I watched a storm system slowly crawl up the east coast over the course of a day. At one point in the evening it began to rain on the map in the New York City area (that's where I live). Just as I was about to take a closer look, rain started pelting my window!
NOAA's 3D weather is mean to be be an education tool but weather in persistent online worlds is an interesting concept. World of Warcraft recently added weather to its extremely popular online game. The WoW weather was also announced in a Blizzard newsletter.
Second Life 
