Imagination!

Textures:
gender@lj/masterjinn@deviantArt
heartdivide@lj
grrliz@lj
gosh@dw
resurgere@deviantArt
lostandtaken.com
silentevocation@deviantArt

Brushes:
Eduardo Recife (misprintedtype.com)
77words@lj
various DeviantArt users (will update with handles when we open up our 'design resources' folder)

Fonts:
Mac system fonts (hi, Futura)
Adobe suite fonts
Google Web Fonts
dafont.com (most fonts from there are of dubious quality, but there are a few gems in the mix)
fontspace (same advice as with dafont)
myfonts.com
...and various other font sites, but these are the main ones where we get our fonts for icons.