I have worked with Selenium and went on to Bromine, which further more can log your results.
The automation is quite easy, with capture replayin FireFox, using the Selenium IDE.
Then you import the script as php into Bromine. From there you run it as many times you want. And you have historic results in Bromines database.
Bromine can be found at openqa.org.
There are other tools as well. But selenium and bromine are very easy to use.
The difference between Selenium and Bromine is, that Bromine can handle requirements, manual tests, automated tests, defects and suggestions for improvement. It runs on apache web server with php and my SQL.
The one thing about it. You have to have monre than one test PC for getting the most out of Bromine.
The Bromine application runs as a server and runs the automated tests on a test pc after your specications. This is very handy for testing on various os and browers.
Another tool is TestGen4Web and PushToTest. But I have not very much experience with this tool.