In addition to ad networks many content sites also join affiliate programs for additional revenue. Performance with affiliate programs can be very erratic though, some sites make a killing with them, other sites make pennies. The issue is that affiliate programs only pay when a user buys something. How it works is you refer a user to a merchant and then get a commission depending on what that person buys (if anything). As such affiliate programs really only work well when your site's visitors are already in a buying mood. For instance a site that reviews digital cameras could do very well with affiliate programs because most visitors are likely already planning to buy a digital camera. In contrast a fan site or blog isn't likely to generate much affiliate income because visitors going there aren't already predisposed to making a purchase.
One benefit from an affiliate program though is that they generally have more implementation methods than ad networks. You can promote an affiliate program with as little as a text link in an article. As such you can use them along with ad networks and any additional money made from the affiliate program is a bonus and does not come at the expense of your ad network revenue.
Many merchants, such as Amazon, run their own affiliate program in house. Others run it through an affiliate network. The major affiliate networks are Linkshare, Befree, and Commission Junction. Amazon's program is great, as are many in house programs, but for a new publisher working with a network will give you more options while you're still trying to figure out what works for your site.
Search Engines value links in their algorithms and many people in their efforts to rank higher will purchase links from sites. So even a site without much hope for ad network success can make money by selling link placements. You should know however that search engines look down on this behavior, especially if the links are for completely unrelated sites. So if you sell a link to an unrelated site you are risking a potential penalty from search engines.
Prices are generally based on your Google PageRank. PageRank or PR, is an approximate value if the total weight of the links pointing to your site. If you use the Google toolbar the PageRank of every page you view is displayed in a little green bar. Two services for selling text links are Linkadage and Text Link Brokers.