With my 12 Days of CSS-mas, I’ve had need for patterned backgrounds made from pure CSS. I’ve found a couple of great sources which seem to cover the gamut of possibility.
Lea Verou’s extremely popular CSS3 Patterns Gallery (also on GitHub) has pretty much every pattern you would need, and you can edit them right in the page to adjust to your liking. This is my go-to for CSS background patterns.
TechGlimpse’s editable CSS pattern gallery contains about half a dozen other patterns that aren’t in Lea’s gallery. Maybe I should do a pull request…in my spare damned time amirite? =D
An honorable mention is Patternify, although this utilizes data URIs by default and that extra HTTP request isn’t ideal in many situations, this also allows you to create your own patterns and save them as a PNG.