This is one of a few particle generators I have. I used this on the thatworks site and also made an earlier posting of it for forking on wonderfl. There are a fair few parameters you can tweak to get your rain looking how you want it. Make sure you have the latest versions of TweenLite available from greensock. You can make the rain start and stop periodically or just have it constantly going... see the attach method for these options.