Selenium is widely used, mostly because of their recorder (which is supported only in FF, but you can then replay the tests hitting IE). Its language is commonly referred as "Selenesse".
It's also a great choice if you expect to do some performance testing in a grid... but it seems this is not your case.
However, I would recommend not to use Salenium/Selenesse; instead, look for Selenium + Webdriver (Selenium 2), which is still in beta. The language construct are much better (but you don't have a recorder).
Other good option is Sahi. I found its API to be easier, in the sense that Sahi's code tries to deal itself with the problem of finding the button/link/etc you want him to click. For example, instead of relying in complex XPath expressions, you can say something like "click the link tag nearest to the div with attribute id="menu" (you can do this with one line of code). It has also a recorder.
