Drupal has built-in caching mechanism which is the easiest way to improve performance of your Drupal site. However, in some cases built-in caching does not prove to be enough and this is where contrib Drupal modules comes into play. To make your work easier, i have compiled a list of Drupal performance optimization modules which can be found below.
Memcached module provides integration between Drupal and Memcached. It is a bit complicated to install as it requires installation of memcached software and PHP extensions on your server in addition to drupal module. If you're on shared hosting then you will not have enough permissions to install this software.
You can read more about Memcached caching system here.
Boost module works well in shared hosting environments. However, this module requires a bit of experience with drupal as it does not work out of the box. It is necessary that your site is running on clean urls and has Poormanscron module and clean urls. You’ll also need to configure .htaccess and robots.txt files for the module to work properly.
Varnish module works as a bridge between your Drupal site and Varnish cache system. Like Memcached, Varnish has additional software to install so you'll need VPS or dedicated server.
You can read more about Varnish cache system here.
CDN module makes it possbile for Drupal sites to use Content Delivery Network very easily. It uses drupal hooks to alter file urls so that the CDN version of files are served instead of your server.
You can read more about Content Delivery Network here.
The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code. Besides a opcode cache it provides a user cache for storing application data. This module uses the APC user cache as a cache backend for Drupal.