I second Bat's request. PLEASE let us control how particles are billboarded/rotated. Right now there is no way to create a rain effect because of the way all particles auto face the camera on all axis.
Couldnt be too difficult to add a
set emitter billboard emitterID, Xangle, Yangle
X and Y angle parms would be a bool, indicating if the particle will face the camera along that axis or not.
The rain Gods beg you! I know you can do it Mike!
PS - If you could also include a way to set the x/y dimensions of particles that would be great too. Then you wouldnt even need a texture for raindrops, you could just use a thin rectangular plain (or perhaps even a triangle) that only billboards along the camera y angle.
All you need is zeal