OneVerify has the largest selection of sites anywhere! You can choose from over 1000 different categories and niches. It's high-quality stuff and it's well-worth the low monthly charge to access their always-updating network. For the best AVS, visit OneVerify now!
There are plenty of other systems to choose from, but most have old sites (some dating back to 1996!) and a poor selection of material. There are exceptions, however. I've provided links above to the ones found online. You can choose for yourself what you like the best. Let me know if you find something I don't have on the list.