Check out this tutorial that shows how to create water drops running down a plane that uses nParticles in Maya.
With all the new features in recent versions of Autodesk Maya, you have more options than ever when it comes to creating effects. Take for instance creating water drops – you can use MASH or xGen, or you can use the old tried and true nParticles system. Check out this tutorial from the Simple Arcade Gamers channel on youtube that shows how to create rain on glass water drop effect.
The tutorial covers working with expressions to make the particles to the right size, shape, and motion for water drops running down a glass pane. Keep in mind that some scripts can help create water drops easily, such as Zeno Pelgrims’ Water Drop Generator which will place water drops on the surface of an object.