Performance

Speeds up the querying, by name, of terms in a taxonomy, by stopping code from having to use the SQL "LOWER" command.

The Beautify module buffers the HTML output of Drupal and processes it through a formatter (e.g. custom set of built-in search/replace functions, Tidy HTML extension or htmLawed script.

Alter cache settings per block. Cache settings per block are now set in code, but if you don't like the default - usually none - you can now easily change this.

The path caching module caches URL aliases to prevent Drupal from looking them up in the database.

This module allows users to specify a preferred bandwidth for viewing videos.

Queues email sending from your site so that instead of being sent immediately it is sent on cron or via some other queue processor.

Useful for high traffic sites that can send a lot of emails on individual page requests.

Drupal 7

The Drupal 7 version supports sending with any email framework, i.e. it works perfectly with Mime Mail module or any other of framework that handles the actual sending of emails. It also uses a standard Drupal queue to do the queuing of emails. It requires at least Drupal 7.12.

The Drupal 7 version is actively maintained.

Drupal 8

The Drupal 8 version is based on the Drupal 7 one and has all the same features/benefits.

The Drupal 8 version is actively maintained.

Drupal 6

Queues any email sent from your Drupal site and then uses the Job queue module to send them via cron. Emailing only supports the PHP mail function.

The Drupal 6 version is in feature freeze, and only serious bug fixes and security issues will be fixed.