The following is a list of free graphics resources available for use.

General

GIMPStuff - A huge collection of resources for GIMP including images, brushes, fonts, patterns, plugins, scripts and more.

InkscapeStuff - A huge collection of resources for Inkscape including drawings, templates, fonts and more.

ScribusStuff - A huge collection of resources for Scribus including templates, scripts and more.

Rastor

Rough Textures - free textures of rough surfaces like granite and concrete

CGTextures - A huge repository of textures for computer generated images.

High Resolution Grungy Paper Textures - can also be used for rough/rock/concrete textures

Huge Collection of Textures - lots of variety here.

More Textures - Lots of stuff here, paper, wood, rock, brick, metal, gritty, grainy, water and more.

Seamless Textures - A large collection of seamless textures, great for modeling or rendering.

Metal Stuff - Large collection of metal textures, brushes and Photoshop tutorials.

Vector

Free Vectors - There's a wide variety of stuff in there, but some of it is very useful.

Free Vectors - a lot of abstract stuff in here, but lots of variety.

Brushes

Photoshop Brushes - A collection of free brushes for Photoshop including trees, plants, clouds, cityscapes, and more.

Models

 The Official Blender Model Repository - A compilation of models for Blender

Boston City Models - 3D models of the city of Boston from the Boston Redevelopment Authority.