I just ran across one of the most useful docs I've seen posted by the team on DevCentral. If you're new to iRules, possibly looking for ways to expand your knowledge, or just bored and looking for something cool to do, this document in the iRule wiki is a must-read. It breaks down some of the most common ways iRules are used and it provides some really useful building blocks for when you want to combine elements for that "double black diamond"-like challenge you're facing.
For example: how about an iRule that simply illustrates logging (a useful debugging technique in itself)? Or, trying to limit pesky client attempts to throttle your servers with excessive connections? Maybe you're interested in using Data Groups (class files) that allow you to abstract variables in a better, more organized, and efficient manner.
Check this out.
(btw - a special nod to Colin for this - great work!)