Amm like the penguin above me said, Bing is the best 3rd option. But why would you quite Adwords? They have the widest cover.
I heard that MSN Ad Center is very good after Google adwords but I haven't tried it. So you can consider them also.
Check out hightrafficacademy.com, register for their free training, and on the video 2 (if I remember that right) you can download a list of great PPC networks (30+ networks).