By my oppinion most flexible is adding a PPC Ad on the website.
We just register an account with company like Google Adsense and place the right sized Ad spot code on the website. Then content relevant ads are served to our visitors.
There are PPC companies like:
Google Adsense
Adbrite
Bidvertiser
The best is Adsense, but they are not for adult websites and are quite strict.
What is the most flexible and fitting website monetisation in Your oppinion?