If you need a simple rotating ad (e.g. banners, text blocks or any other HTML code) at the bottom of every post, you can use this simple freeware plugin:
http://wordpress.org/plugins/wp-quote/
Just your ads as raw HTML lines (one ad per line) into WP Quote box. E.g.:
Code:
<p align="center"><a href="http://www.mysite.com/><img src="/mybanner.jpg"></a></p>
<p align="center"><a href="http://www.mysite2.com/><img src="/anotherbanner.jpg"></a></p>
<p align="center"><a href="http://www.mysite3.com/>MY TEXT AD</p>
And vualá!
