There are many alternatives to Google AdSense and I am going to go over the most popular ones.
AdBrite was a great alternative to AdSense. AdBrite use to be the number one alternative, at least in my eyes, but as of February 1st, 2013, AdBrite shut its doors due to a business deal that fell through. This would have been one of the only ones that I suggested but I guess I'm going to have to be unbiased here and suggest some other companies that will also do the trick.
Another program that I would suggest would be Bidvertiser. Although they are a little smaller and their ads aren't as flexible as AdSense ads, the rates are pretty good. Plus, you can cash out through PayPal! This is a hard thing to find with Pay-Per-Click programs such as AdSense.
Chitika is also a great alternative to AdSense. If a visitor used a search engine to find your website, Chitika will deliver them ads based on the search terms they used. That is a big plus of the network. But there is a downside. This network is more for websites with a lot of traffic. I recommend using them only if you get a lot of traffic and if you sell stuff from your website. It's kind of like being an Amazon affiliate so Chitika is best used with a specific type of audience.
Clicksor would be another way to go. Clicksor has been around for a while. Their ads are pretty nicely displayed and they look a lot like AdSense ads so it's pretty high quality. They offer quick approval of ads and have a great support team behind them if you run into any trouble or have some questions regarding their program. Their rates are also decent.
InfoLinks seems like another great AdSense alternative. Although I haven't tried this program myself, I have ran into a lot of people that have and they all seem to love it.
Now if you have an adult related website then you can try AdultSense. It's an excellent "xxx" Pay Per Click program that is the adult rival of Google AdSense. AdultSense's rates are great and the ads, although all extremely adult oriented, are of high quality.
There are many alternatives to Google AdSense but the above Pay Per Click programs are the best ones in my eyes, and a lot of other webmasters. But if you feel different and would like me to add any other Pay Per Click programs to my little list here, please feel free to use the Contact Page above and email me them.