Often there is a need to insert ad code (or other bit of content) directly after the first post on a page, but not after any others. You can do this by adding a counter and an if statement to the loop.
This site is managed by Randa Clay who got tired of trying to keep track of all the cool solutions, ideas, scripts and bits of code that one day might be needed.
This is much better than searching around on Google and rummaging through my bookmarks for that thing I read a month ago. I hope you will find the site helpful as well.