Let's start by making a new Layer (CTRL+SHIFT+N) and Name it "rain drops".
Select the Brush Tool (B) and Set Foreground Color to White,
Start dabbing the Rain Drops Layer with the brush to create the basic shapes of the drops (Varying your Brush sizes)
Tutorial Step 2
Now use Photoshop's Filter>Distort>Ripple and Set ammount to 50% and Size to Large.
What you're doing here is changing up the shapes and making them appear more random, not just a bunch of perfect circles. So you can change what % to Ripple it depending on your taste.
Tutorial Step 3
With the Rain Drops Layer active, Lets use Photoshop's Styles to finish the drops. Goto Layer>Layer Style>Drop Shadow (Settings), Now Switch over to Inner Shadow (Settings), Now Inner Glow (Settings), And finally Goto Bevel & Emboss (Settings).
Tutorial Step 4
Now all thats left to do is goto the Layers window in photoshop and set the "Rain drops" Layer's Fill Opacity to 0% so only the Layer's Style is seen. This looks more impressive when it is layed over something other than a flat gray background. Straight drippin wet mutha fucka.