A crucial part of modern (post 2009) design is the use of textures in graphic design to make objects appear more tactile.
Although pretty easy to accomplish in Photoshop, Illustrator requires a different tack that when mastered can deliver amazing effects.
This tut by Ryan Putnam, the guru over at Vectips, shows you just how to do it.

This is a really quick and easy tutorial using textures, gradients and the blend tool to create an abstract piece of art.

Lost and Taken is a great collection of textures made freely available for use in both personal and commercial projects including web templates, designs, and other materials intended for distribution.