Stain brushes are extremely useful for any digital artist. Photoshop stains can be used to add realistic stains on different surfaces like paper, wall, floor etc. But the stains Photoshop brushes can be used in more artistic ways to give a grungy effect to an illustration for example.
The most commonly used are the coffee stain brushes, the tea stain brushes and the blood stain brushes. I have tried to gather lots of different brushes so that you find your right stain brush here.
If you don't have Photoshop, you can use MockoFun which is an online graphic design software that you can use for free. It's a great Photoshop alternative.
In the MockoFun library you can find lots of stains, splatters and splashes that you can blend with your photos. The stains images from MockoFun are in PNG format, so they are quite easy to use!