While playing with some friends today, I thought why not have seasons on minecraft, so it starts snowing and raining at certain times, and there are times when it's just warm or cold. Though since ice doesn't melt if there's no snow, sometimes the map generator even generates areas with ice and snow and then a desert right next to it, we'd need to make it melt somehow. I'm pretty sure this isn't made yet since I searched, but if it is, please point me at the right direction.
There once was rain and snow client-side and I think Notch annouced weather to return some day. This would be mostly a client mod with the server for synchronization, though.
Notch said that he would introduce snow and rain back into the game in Beta 1.5, also adding lightning. Here is a video of him testing it, as well as all the other features coming also with the new update.
