I was using a free CloudShare pro account, and they provided operating systems (including Windows 7, Windows XP, and Ubuntu) which I could access from anywhere in the browser.
Are there any other services like this?
You can try Eye OS. And its free as well! Doesn't let you run Windows, Linux or any of the other operating systems though.
I don't think there is anything that is completely free, but Amazon EC2 can be very cheap if you are not using very much bandwidth and you only need it for limited time. It would depend on what you need to use it for.
Do you want something that you want up and running 24 hours a day or something that you can use to test drive? EC2 will charge $0.09/hour for Linux and $0.12/hour for Windows. (as long as bandwidth is less then 1 GB/month)
BrowserCam has a number of offerings for testing websites/programs on different OS's.
They range from free screenshots, to full remote desktop access.
It has its own OS