There are a number of useful Drupal modules quietly working hard at the backend on websites around the globe. They usually come without any visible UI to the end user. You should get to know about them. In alphabetically order:

1. Backup and Migrate
   Simplifies the task of backing up and restoring your Drupal database or migrating data from one Drupal site to another, with automatic scheduled backups.
2. Boost
   Provides static page caching, enabling significant performance and scalability boost for heavily-trafficked Drupal sites that receive mostly anonymous traffic. forDrupal demo sites are running with Boost.
3. Cache Router
   Another caching system for Drupal allowing you to assign individual cache tables to specific cache technology - APC, Database, eAccelerator, File, Memcache, XCache
4. Checkbox Validate
   An 'install and forget' module. Corrects display and behaviour of required checkboxes in all Drupal forms, which is a core bug.
5. Global Redirect
   Another 'install and forget' module. Automatically maintain single path alias and perform proper 301 redirection. Good for SEO.
6. Mollom
   One stop solution for all spam problems, almost. Protects comment form, contact form, user registration form, password request form and any node form.
7. Poormanscron
   A module that runs the Drupal cron operations without needing the cron application.
8. String Overrides
   Provides a quick and easy way to replace any text on the site, anything that's passed through t() actually.
9. Rules
   Define conditionally executed actions based on occurring events, support flexible scheduling system, a replacement for trigger and workflow-ng.
10. Domain Access
    A group of lightweight modules that provide tools for running a group of affiliated sites from one Drupal installation and a single shared database, allows you to share users, content, and configurations across a group of sites.

What's your favourite backend modules?