WordPress has some really good plugins that can make a site load much faster, here are some  that will aid in improving and optimising your sites performance.

WP file Cache

This plugin implements object level persistent caching and can be used instead of the built in WordPress WP_Object_Cache. It will also aid in significantly reducing the load-time from your database. However, it should be noted that WP File Cache shifts the load from your database to your disk/file system and if Disk I/O is a bottleneck, file based caches will not help you.

Quick Cache (Alternative to WP Super Cache)

This is a very useful plugin when it comes to optimizing your website’s speed. Quick Cache takes a real-time snapshot (building a cache) of every Page, Post, Category, Link, etc. These snapshots are then stored (cached) intuitively, so they can be referenced later, in order to save all of that processing time that has been slowing your site down.
It uses configuration options, that you select from the options panel.
Quick Cache uses advanced techniques that allow it to recognize when it should and should not serve a cached version of the file.

W3 Total Cache

This plugin helps to improve the sites’s performance via caching: browser, page, object, database, minify and content delivery network support. It improves the user experience of your site by improving your server performance, caching every aspect of your site, reducing the download times and providing transparent content delivery network (CDN) integration

 

WP Minify

This plugin integrates the Minify Engine (a PHP5 app that helps you to follow several of Yahoo!’s Rules for High Performance Web Sites) into your WordPress blog, to combine and compress JS and CSS files to improve page load time. Some of its features include – debug tools, plugin hooks, support for minifying external files via caching, ability to pass extra arguments to minify engines etc.

Parallelize

Parallelize allows automatically parallelizing WordPress attachement files (images or any files uploaded with the wordpress media features) across multiple hostnames, speeding page load. To assist caching, resources will always load from the same hostname.

WP smush.it

This plugin reduces image file size and improves performance using the Smush.it API within WordPress. Every image added to the page is automatically run through Smush.it on the fly.

WP HTTP Compression

This plugin will allow your WordPress blog to output pages compressed in gzip format, if a browser supports compression. However, it should be noted that this plugin should not be used alongside WP Super Cache or WP Cache as those plugins are already implementing compression. Your blog will keep working but those plugins will stop caching new pages.
HTTP compression generally means a 60-80% REDUCTION in the size of your pages (broadband usage) as well as an INCREASE in download speeds of 3x to 4x.

Hyper Cache

This plugin is flexible and easy to configure cache system for WordPress. This is a new cache system for WordPress, specifically written for people who have their blogs on a low resource hosting provider. It can work even with hosting based on Microsoft IIS. Its features are easy to configure, easy to integrate with other plugins, global translator compatibility, redirects caching, auto clean system to reduce the disk usage and more.

WP Super Cache

This plugin generates static HTML files from your dynamic WordPress blog. After a HTML file is generated, your web server will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts.
99% of your visitors will be served static html files. Those users who don’t see the static files will still benefit because they will see different cached files that aren’t quite as efficient but still better than uncached.

DB Cache Reloaded

This plugin caches every database query within a given lifetime. It is much faster than other HTML caching plugins and uses less disk space for caching. DB Cache Reloaded ensures that a site works much faster and uses less disk space for cached files. It also makes sure that visitors always get actual information in sidebars and server CPU loads are as low as possible.

CDN tools

This plugin is designed to help you drastically speed up your blog’s load time by loading data onto a content distribution network (CDN). It allows you to load Javascript and media files to an external server to drastically speed page loads. You can side load data to a commercial CDN or just load your larger JS libraries (Prototype, jQuery) for free from Google’s servers.