I just saw ClickTale in delicious. It will allow webmasters to view movies of their users' behaviour on their site, right down to mouse movements.

There's a lot of potential here. It could be a real help in isolating bugs. When a user writes a complaint email, match his IP/cookie to the profile and just watch exactly what he did. Then even those myriad "i went to ur website and it doesnt work" would be become useful.

And mouse tracking alone, wow. I can imagine in the not too distant future, I let a shaky old person take the keyboard and when I get it back, the ads on all my favourite sites are related to Parkinson's Disease. When they see the mouse get up to speed again the ads may ask me to write a testimonial for their miracle cure.