WebPageTest is a great tool for testing and analysing how quickly web pages load.
Many people just use it as a simple testing tool, but it has advanced scripting capabilities for multi-page testing, completing forms etc.
It also has an API so performance testing can be integrated into Continuous Integration processes, used for monitoring and analysing how the web is built.
These slides explore some of these capabilities in more detail.
There are bonus slides after the "Thank You" slide