Entirely free is Google Website Optimizer which is pretty quick to get going with, but also pretty comprehensive. There's a couple of books available from Amazon if you should need them, and quite a few sites out there giving hints on it. As far as I know it's the only free one, but does full multi goal multi-variate testing, albeit not with realtime results.
There's a website which lists all the known multi-variate tools and compares them here: http://www.whichmvt.com/
Check out http://visualwebsiteoptimizer.com/ I am using it since last week. The set up is very easy and intuitive and support is awesome, at times you may get answer within 2-3 mins of sending an email.
Unlike GA it provides real time reporting which is a boon when you are in midst of huge incoming traffic.