Harrys Graphics Pack
... contains 15 seamless textures.

Renfield Imaging
Great textures from one of the winners of the Galaxy 2000 Contest.

Cybia Creative Development
Lots of texture for Photoshop and PSP

Texture Synthesis and Analysis by Rupert Paget
Fast texture synthesis by nonparametric Markov random field modelling for synthesis of natural textures. Source code is provided, plus many free textures.

EYE Textures
Hundreds of different textures

Tao Tao Textures
150 Sample Textures

IBM-Ease of Use-Textures
Dozens of natural textures in different resolutions for free