Single raindrop sound effect
I've looked, but am not able to find, a recording of the sound of a single rain drop. You can download any one of them and use a free audio editor (like Audacity) to trim it to the sound of one drop.
If you really want one, has a lot of raindrop sounds.
You're hearing louder and softer sounds (due to raindrop size or speed, or due to nearness to the observer) and impacts on a variety of different surface (water, cement and leaves, for example, would all sound different). You might come close if you have four or five samples and assigned each one a random timing and random volume level.
For slideshow, Powerpoint presentation, ring tone, videos, audio books, flash games or just for fun with your MP3 player, pc, laptop and PDA/tablet.
Vicey-versey, people see a raindrop hittin' with a 'kerr-plup', an' they'll assume that be the actual real sound. You can download any one of them and use a free audio editor (like Audacity) to trim it to the sound of one drop. Rain, I guess, only gives the impression of rain as a collective.
Rain as a collective has that sound because of the variation from thousands of drops.