SubtlePatterns is a website which has a massive library of background patterns that you can use for free on your own website. This is the place to go if you are looking for a nice background for any website. There are currently 316 of patterns to choose from for anything you will need, from light and dark backgrounds and everything inbetween. To view some of the backgrounds available to you just have a quick look through the SubtlePatterns website.
But this article is to let you know about an awesome tool which uses SubtlePatterns, this tool is the SubtlePatterns bookmarklet created by @bradjasper. This is a really useful tool that will allow you to preview any SubtlePattern on any website. So if you are looking for a new background on your own website instead of browsing SubtlePatterns for a couple of patterns you like, download them, upload the images to your server, change the CSS to preview the new background, then repeat for all the images you want to preview.
Try it out on your website. SubtlePattern Bookmarklet