A while back I mentioned that I was working on an addon for OpenOffice called ooregress, which adds some more advanced statistics functionality to OpenOffice Calc.
Unfortunately when I wrote it, it was for OpenOffice 2.4. I think they changed the API for OpenOffice 3.0, so my code no longer works.
This one is in a website, so all you need installed is a supported web browser. Since this is a personal project, IE is not a supported web browser :P I am currently supporting Firefox, Chrome and Safari - I can probably support Opera too, I'd just have to install it.
A temporary link is here, although I won't guarantee that this will always be active. I've got it hosted on my SheevaPlug, which I turn off sometimes. UPDATE: The site is now statsan.com, and I've called it StatSan for STATiStical ANalysis.
I'm planning on putting all the code as open source (GPL) so that anybody can host their own version of it should you want to, I'll put up the link once I get something set up - I've got a local git repo of it, it's just a matter of creating something on GitHub and pushing everything over.
So if you have to do stats calculations, feel free to check out this app and let me know what you think. If you find any bugs or come up with any features you'd like, feel free to let me know.