The most common tool for producing this effect is a toothbrush.
I like to take my brush and cut off all the bristles except the blue ones. That's two rows worth, for those of you who don't use Oral-B brushes.
If you use a brush with wavy, uneven bristles, you might find that those make it way harder to smoothly thumb the brush later on. A generic, flat brush works best for this purpose.
Here are a few suggested uses for splatter:



As this week's title suggests, I'm not actually that huge a fan of using speckly shit. It has it's place, but just like the gradient tool in Photoshop, it's overused and more often that not tastelessly used. I think it's often a substitute for drawing for people who can't be bothered to actually draw spattering or textures. Plus, I hate how dirty it gets my hand: