We are getting ready to roll out a new service at work, and are particularly concerned about the traffic levels it could receive, so we embarked on a quick-and-dirty load test.
I took a quick look at the open source tools available, and most are only capable of hammering on a URL, or on a list of URLs. Because our application is dynamic, with newly-created unique IDs being passed from URL to URL, we needed more expressive power than a data file of URLs could give us.
Then I found funkload. As the name (sort of) suggests, it can do both functional and load testing, and that is where its power lies. The functional testing focus means that you write your tests as Python unit tests, with the full power of Python and webunit to build the test case. Funkload can run the tests like functional tests (run each once and see if they succeed), or like load tests (run just one, but over and over).
I wrote a test case that runs through the entire experience end-to-end, about a dozen URLs, screen-scraping the returned HTML where I needed to extract dynamic data for use in the next URL. Funkload then ran the test on many thread at a measured rate, producing a report on tests per second, successes and failures, and so on. Looking at the funkload's numbers and the server stats during the test, we could determine the traffic load our servers are capable of.
Funkload isn't the fanciest load tester out there, and if you only need to hammer on a home page, it's more than you need, but for load testing dynamic applications, it's just right.