Both the addons by
@Siropu and
@AndyB are excellent.
It really depends on your needs. The Siropu option allows very precise control over how the ads are rotated and where they are displayed, but it's also complicated to set up and configure so expect a learning curve until you get it right.
The AndyB is simpler (and cheaper) and will do the job nicely if your needs don't require the more elaborate Siropu addon.