Seeing as how I've compiled a fairly substantial list of textures in the pursuit of making models and worlds I thought I'd share some of them with you.
BrickPath
A set of 16 seamless textures you can piece together to form a nice little brick pathway all across your world. (~1.25MB)
CanyonScape
MountainScape (shown above)
and TreeScape
Three panoramic landscape you can use as sky textures. (not quite seamless, could perhaps be made to be)
Metal Textures (5MB)
Fabric and Armor (1MB)
Land and Sky (3.1MB)
Glowy Stuff (0.4MB)
Wood Textures (2MB)
SpaceTextures (0.3MB)
Knight Textures (a group of images I used to make the knight model texture, 2.7MB)