Matt Richardson shares a Nuke Gizmo called RainMaker that can easily create the effect of lens rain drops in Nuke. Add RainMaker to your shot, and set the start frame and drop lifetime to easily place drops on the shot.
You can choose between three different drip sizes and set the drop and gravity controls choosing between attributes such as blob style, warp, and trails for the drops.
RainMaker offers some really realistic results and makes adding lens rain drops in Nuke a snap. You can get the RainMaker Gizmo for Nuke 8 and up over at Nukepia here: RainMaker v1.1 Add lens rain drops in Nuke.