This is a bit off-topic, but if you're a Firefox user, it may be helpful. Like many Firefox users, I've installed many extensions, but since those extensions are essentially separate programs, they bog the browser down. Eventually, I go through and turn off or uninstall all but the essential extensions. However, that doesn't always fix the performance problems.
There's another category of Firefox add-on that most users pay much less attention to--plugins. Media players, such as those for Adobe Flash and Apple Quicktime, are installed into Firefox as plugins, but there are many other applications that silently add plugins to Firefox. You can uninstall an application, but its plugin may be left behind.
To review and clean up your plugins, go to the Tools menu, select Add-ons, and then click on the Plugins icon. Turn off all the plugins for media formats and applications that you no longer use. If you see a plugin that you don't recognize, you can turn it off, and if it causes problems with viewing certain content or running applications, simply turn it back on. Depending on the number of plugins you've got, you may see a significant improvement in Firefox's performance.