SVG Patterns is a great site with many examples of using SVG patterns for backgrounds.
SVG patterns have the benefit of being scalable (they are vector graphics) while typically being smaller than bitmap graphics. They also are have more browser support than CSS3 gradients.
The site links to code samples for each pattern on Codepen so you can test them out, fork them and play with them, etc.
The idea behind SVG patterns was inspired by CSS3 Patterns, which is pretty cool.