(Disclaimer: I'm still just learning this stuff, so please let me know if you notice anything wrong in here)
If you build and maintain Drupal sites and haven't used Drush yet, it's time you take a look at it. Even if you only use it for very basic things such as downloading and updating modules it can still be a big time saver.
One of the things that makes Drush so powerful is it's flexibility with hooks. Each Drush command can be hooked in at three different levels; before, during, and after it runs. The hooks are pre, validate, and post, respectively. It works slightly differently than the Drupal hook system but the concept is the same.
hadfield on Features Part 2 - Managing your feature
Capi Etheriel on Features Part 2 - Managing your feature
Capi Etheriel on 9 Drush links to take you from beginner to advanced.
Avery St. Clair on Nobody NEEDS a cellphone
Anonymous on A review of my Netfone VoIP service
Fred on A basic install profile