Google and Yahoo-Overture control over 90% of the PPC market and as such you should seriously consider using them if you want the maximum possible exposure to targeted web traffic through PPC advertising .
Sign up for the Google Adwords program is free but they require a $5 USD deposit before they will start to display your ad copy. Google provides an excellent interface to work with. A pleasant modular design groups all of your work into nice "containers" that can be manipulated and viewed in a variety of ways.
The system reporting is by no means real time but the delay on their results display is palatable when comparing them to the other major PPC provider, Yahoo-Overture.

The general public probably best recognizes Google
as the web's premier search engine. Google's dominance
on the Internet can't be denied or argued. But
perhaps what many people don't realize, Google has
become a premium revenue source for many webmasters
and online marketers.