1) Google Adwords
Google adwords is still one of the top dogs. Yes, it is true that it is becoming more competitive and expensive, but it's still hard to ignore. Why? Because it has the most traffic.
However, traffic means nothing if it doesn't profitably convert. Therefore, it is important that you are sure that your ads convert and that you are paying a proper price for advertising before you go crazy with Google. A good thing to do is test your ads at more affordable places before you do a big adwords campaign.
2) MSN Adcenter
For some reason MSN gets ignored amongst the Googles and Yahoo's; however, it is probably one of the best places out there. It still has great traffic, and, what sets it apart is that it is usually more affordable and less competitive. These characteristics make it a must for any good PPC campaign.
3) Yahoo Search Marketing
Yahoo is another place to consider for similar reasons as Google: It has great traffic. Yet, also like Google, competition and high prices have made Yahoo a bit less attractive. Again, before you go nuts with Yahoo, make sure you have tested your ads affordably and fined tuned them into conversion machines.
Although not the traffic monsters as the big players, Searchfeed can be a great place to start testing ads at decent prices. The conversion rates can be good once you get a feel for the type of traffic you are getting. You can check them out at Searchfeed.com.
Similar to Searchfeed, Looksmart also has less competition and affordable prices. It's been around since 1997 and has been known to get some pretty good conversion rates. Odds are it will continue to be a good option in any PPC campaign
6) Search123, Ehance, Miva, Adbrite
If you don't like the ones mentioned above, try any of these 4. They will all vary in prices, traffic, and conversions, but depending on your offer, one might work better than the other.
The key to successful PPC is testing and trying new things. Therefore, it is important to try more than one PPC network. If you are just starting out, it might be best to try the smaller networks and work your way up. This way you will learn a lot, and not blow your budget. Good luck!