Subtle Patterns: hundreds of free backgrounds that tile
Heads up! This post was written in 2014, so it may contain information that is no longer accurate. I keep posts like this around for historical purposes and to prevent link rot, so please keep this in mind as you're reading.
I've been using Subtle Patterns for quite some time now. All the patterns are great, and their Photoshop plugin is incredibly useful.
I can't remember when it first launched, but I do remember when there was only one or two pages to browse on the site. Now it has over 380 images you can use for free in your own projects. The collection is curated by Atle Mo, who does a great job in maintaining subtle, consistent backgrounds out of the tons of images that users submit.
All of the patterns tile nicely (which you can test by selecting Preview) and all are available under a Creative Commons license. If you're a designer with a knack for tessellations, you might be one of the lucky 3% whose pattern gets accepted.
I bought the Photoshop plugin the day it came out, which I highly recommend if you use Photoshop for web design. It's so much better than tiling the images yourself or going to Edit > Define Pattern every time you want to try out a new background. It also has a slick logo, which always a good reason to buy something.
Seriously, it's an awesome service, so now's a good time to add Subtle Patterns to your bookmarks if it's not already there.