Subtle Patterns has page of interesting images that are perfect for seamlessly tiling the background of your website. Clicking the preview button that appears when you hover over a sample let's you quickly see what it looks like to have the pattern applied as the background of your site.
Strip Generator has been around for a long time but it's still one of my favorites. You can use the generator to create any combination of strips, from fat, to thin, from colorful, to black and white. When you're happy with a look download it, and use it to tile the background of your website.
Colour Lovers is not only a great place to generate color palettes for your site, but also backgrounds. Who knew?
Why do I like TransparentTextures.com? Because they're transparent. The transparency lets me change the color of any texture and apply it to the background of my site.
5. Grohlf.com/trianglify-generator/
I thought I'd throw this one in here because it's a little different. It takes all kinds of triangles and colors to create an interesting textured background from yours site.