Graphic Resources:
Afterglow Textures: Huge selection of textures and flats to use in your own maps.
Doom Vault: Another good source of textures, flats, and skies.
Image Analyzer: Great free program to resize, recolor and convert graphics.
Use with any simple paint program to make your own textures.
Doom fonts: (24KB) Five red and grey fonts made for use with Bitmap Font Writer.