Being, me, I've been surfing the Net for years. During that time, I've come across a lot of webdesign and background sites, although I've lost most of them into the depths of our emptied-out cache... These are the ones I still remember.

The Pixel Foundry
Backgrounds, images, and so forth.

Free Webpage Backgrounds
At least, that's what my Bookmarks say. I'm not sure what the site's actually called...

Backgrounds, Textures, Icons, Balls and Page Dividers
Just that.

ClipArt Castle
All sorts of images, backgrounds, etc etc.

Texture Land
All sorts of textured backgrounds.

The Background Sampler
The Netscape home page's supply of nato backgrounds. Fabric, marbled, rock, this place has a heap! Warning: many images on one page. Takes a long time to download!

I'm always on the looout for more, so watch this space. And if anyone knows of any more, email me at websites@alkalign.htmlplanet.com.