Look again!
It happens to me all the time, and I'm sure the same thing is happening to you. You need some specific functionality for your Drupal site and the developer instict kicks in. That is, the unavoidable thought "I'm sure there aren't any module available that does this - I'll develop my own custom module!". Have you experience this too?
The lesson I have learned again and again is that the amount of contributed modules for Drupal is truly astonishing. If you can't find the module you're looking for, then look again. If you still can't find it, look once more. You should probably look around at least five times before embarking on developing your own custom modules!
This happened again for me, for the umptieth time just yesterday when working with this very website. I was trying to get some verification tags from affiliate networks and Google into the site and was pulling my hair on getting that done in a good way. Enter the little devil (i.e. hacker) in me, convincing me that I have to create my own custom module. After burning a bit of time getting it done, I discovered that there already is a contributed module that does this very thing, does it better, and does a whole lot more. The module in question is called Nodewords and is another little gem in the universe of Drupal modules.
Another lesson from my struggles with existing vs. modules-needed-to-be-written is that the name of a module does not always reveal what the module does or what it can do. So, you probably need to search around on drupal.org and Google for some time before you find it. Also, I have stumbled across really useful modules that are not featured on drupal.org, which means that searching there may not be enough. Still, whatever you're looking for, there is a fair chance that someone else has already built it!
