The networks listed below are the top 3 PPC ad networks (other than AdSense). While not as profitable as AdSense, they are all trusted and respected by the majority of webmasters online.
AdBrite, while not a true PPC network, offers a different and unique concept to advertising online. Webmasters are able to sell ad space on their website and place it in the AdBrite Marketplace, browsed by thousands of advertisers. Obviously, for this to be profitable you'll need a fairly popular website with at least a couple hundred unique visitors a day -- you can, however, use Run-Of-Network (RON) ads to fill this space while you're waiting for advertisers to buy your ad space.
Yahoo! Publisher Network
Yahoo! Publisher Network is still currently in it's beta stages and is only available to webmasters residing in the United States. It is known as the best alternative to AdSense, however there are some known issues with it's contextual ad targeting. Since Yahoo! is a known and reputable company, payments are always sent and on time.
AdEngage is very similar to AdBrite, however they offer much greater flexibility when designing the ad for your website. The advertiser base isn't as large as other network's, so you'll usually find that duplicate ads will appear when using their RON ads. Payments are sent on time and are offered through a range of different options, including PayPal.