Here are a few to think about (as it happens, from the "Resources" appendix of my book :-)
FitNesse: http://fitnesse.org/ FitNesse is an acceptance testing tool that allows tests to be expressed as tables of input data and expected output data, described in Fit for Developing Software: Framework for Integrated Tests.
Watir: http://wtr.rubyforge.org/ Watir is an open source library for automating web browsers allowing automated testing of web applications. It started out on Internet Explorer on Windows but is in the process of being ported to other browsers.
And this blog suggests that while Selenium itself may have some issues with popups, Selenium RC handles them better.