3. Brushes
These were really popular in old school icons and everyone used them back in the day a few years ago. I know I used to see them everywhere, especially on icons depicting couples or cute anime girls. Ranging from grids to foliage, they were a popular way to fill up space and achieve a stylish, "tech" or girly icon. Nowadays the norm has changed slightly, with font and grunge brushes being used as secondary effects and more often than their primary counterparts, the flowers, clouds, butterflies etc. Basically, they've become a sort of second-hand textures.

I admit that I have little experience with brushes in icons since I didn't like how small they had to be to fit in that 100x100 limit - and what's the point in using a flower brush if you can't even tell it's a flower, anyway? Therefore, I lean towards the "second-hand textures" side more: I recommend using celestial brushes (bonus points for more complex ones that also feature stardust!); aside from that, I'm an adept of simplicity and don't like cluttering icons with unnecessary things. Just paste your brush and play with sharpness and contrast, you can get many good textures this way.

4. Grunge textures
I could include all types of textures here since they're all useful, but I want to talk about these since they're the ones I use most of the time. When adding texture to an icon, grunge textures are good because they're relatively easy to make: just take an image, shrink it to 100x100 and sharpen the hell out of it. Voila, a rudimentary texture.

Use these sparingly and make sure you don't hide your character/base image under too many textures; these are supposed to help your icon, not obscure it. As for BMs, I prefer the Dark Trio (dibs on the band name!) Multiply, Color Burn and Linear Burn - remember to delete the parts that look too dark or burnt, not every part of the texture is always usable!

Here are some people and websites that have good textures which I normally use: Acidic Icons, Celestial Star, CrazyKira Resources, Hybrid Genesis (also, their LJ), ICONatic, Icon Dreams, In5omn1ac, Jordanna Morgan, Proverbial Sunrise, Sanami276, Seynee, Station.

...And let's return to internal stuff, this time filters and blending modes!