In the past I've used Muvar & Google, like Ryan said they've both got their good parts and if you need to do things on the cheap they're both adequate.
Then on Tuesday I found a very interesting script that uses the good old AIDA (Attention, Interest, Desire, Action) copywriting formula to test your salesletter...
Rather than just reporting on conversions, it categorizes each visitor into which stage of the AIDA formula they made it to.
E.g. The Attention stage is 20 seconds. If the majority of people leave before this stage then you need to work on your headline. The Interest stage is 1 minute. If this is your sticking point then your subheads, lead paragraph and first page or two of copy probably need attention. etc.
This way you can test your salesletter with much fewer actions, meaning you spend less money on driving traffic before you get your site optimized, and start making serious money faster.
A little hard to explain just in words, but you can see screenshots of the output and get it from . (That's my aff link
)
Anyway that's just one of the features of the script (it's called Conversion Prophet) and there are plenty more that I'm yet to even explore. I've literally only just installed it and still figuring it all out (I got the pro version), so far I'm very impressed.