Program:Photoshop CSToday,i'll show you how you can create a water drop
Step 1: Open an image(any image you would like to have water drops on it.I used an apple.
Step 2: Create a new layer and select the elliptical marquee tool,make a small circle,see image below,it doesn't have to be a perfect circle so don't worry.


Step 3: Select the gradient tool(be sure that the background/foreground is black/white),we will insert a black white linear gradient.see images below.

Step 4: Click on the top-left corner of the selected area and drag it till the bottom right.(follow the red line,i made the red line to show you how you should drag the gradient tool)
Step 5: Deselect and now set the blending mode from normal to overlay

Step 6: Now right click on the layer>blending options.We are going to put a drop shadow and an inner shadow(You can change the properties of the shadows as you liked them best),check images below
this is the result so far:
Step 7:Change the foreground/background to white/black and select the brush tool with brush size around 5-6pixels,i used a 5 pixel brush(it's up to how big your drop it is or not)and make a small dot on the top-left of your water drop.
And ta da...finished!
On another picture,can you guess the fake water drop?


Some tips for better results: If you don't like your result much,you can play with the liquify,it's under the filters(be sure not to use much pressure on it though),you should use the forward warp tool,the pucker tool and the bload tool


here's the result with the liquify
or you can play with the smudge tool