module ratings

Drupal is like a six thousand blade Swiss Army Knife - somewhere in there is exactly the tool you need, but you risk never seeing it in the crowd. A lot of people don't realize there's a critical difference between image.module and imagecache.module. And what's image_attach or easy_image, anyway? There's dozens of node hierarchy solutions out there; I used to use PAC tree, now I use Node Relativity, and soon I'll use Outliner.

So tell us: What module do you use to provide functionality we've never heard of? What tool makes your day with Drupal a million times easier?