Web Development Performance Tools

DipStick McSwindler

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Trojan

For front-end page load times these might be helpful: webpagetest.org, YSlow, Google Page Speed, Cacheability. The first case-study here might be of use too.

For back-end, e.g. tracking CPU usage, etc, you're going to be looking at performance on the particular system. A virtual machine that you can replicate and test with different settings might be useful. Tools include dtrace, truss, strace, Apache Benchmark. I'm sure there's plenty more - this isn't my forté so hopefully some of the folks who do it regularly can chime in.

Don't forget to test on different platforms, e.g. Apache vs Litespeed, etc.

Fussgangerzone

Trojan's recommendations are very good.

There's a free browser plugin called Selenium. It's a browser automation tool which can be used in testing.
It could be a good one for you to investigate because it's free, but it's also used at the enterprise level for finding bugs and errors.
It's commonly used to find errors in large web applications and can be used to test any number of conditions.

Exception trackers could be a good one to look into, too. You may be aware of when an application can "throw an exception" or in common terms, encounters an error. An exception tracker lets you easily monitor these in a nice interface, rather than going blind trawling through log files.

Sentry is one example.
Exceptional is another. This one was actually created by an Irish team, then sold. Their new thing, Intercom could be worth looking at, too.

Bob_Latchford

I would have thought, Selenium would be more functional testing rather than performance or non functional testing.

If your on java you would look at some sort of profiler, jvm etc.
http://java-source.net/open-source/profilers

Heres a live demo of a commercial tool, will give you an idea of metrics used.
http://demo.fusion-reactor.com/fusionreactor/fhtml.cfm

It depends very much on the stack you use as its going to be a lot of measurements and log analysis. For apache you could enable mod status for example, but for iis it might be something else

http://httpd.apache.org/docs/2.2/mod/mod_status.html

if your using a database then that needs monitoring to

DipStick McSwindler

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I do this for a living. We use HP LoadRunner to test web applications. We also use DynaTrace when testing Java applications. It allows you to go down to the code level and see how long pieces of code take to execute. Pretty great!