Drupal modules I use and depend on

I’m just starting to list the Drupal modules that I use on a regular basis, or that I’ve stumbled upon and may well use in the future.

My current “pass criteria” for modules when I employ them on production sites (i.e. will I stake my livelihood on this module?) are as follows…

1. Does it have some history? Has it been around for a while, had a lot of use, and been upgraded to a major Drupal version or two?
2. If it’s a big module, does it have an active community of users and developers?
3. Does it play nicely with other modules, particularly the important ones?
4. Is it easy to configure and hard to mess up?

Where possible, I’ll try to post use cases and personal experiences with these modules. Feel free to add your own experiences or suggestions too.