Blogher is one of the older ad networks in my list.It was started in 2006 to cater to the blog monetization needs of women entrepreneurs running small businesses. It is a great tool for women entrepreneurs. And it is an ad network of choice when your website has women as the primary audience.
You need to sign a one year contract with Blogher, when you signup.They pay users monthly by cheque or PayPal when you reach $25. It is quite flexible as compared to Google ad sense etc as it does not have very stringent minimum traffic requirements. But on the other side, it requires you to have their visual ads on top of all the pages and posts.