There are some situations in which you have to test some asp.net website and there is no webserver installed on work machine. I got in a situation like this when I was updating my cruise control scripts and got no webserver installed on the development machine. I couldn't risk it to directly work in Production environment. Installation of IIS could take a few days due to necessary approvals. But who has time??
UltiDev proved to provide a relief in such situation by providing a server for situations like this. You just have to point to your main page in a directory on your computer and UltiDev will host it on your local machine. Not only you could access it on your computer but from network as well.
You can download Cassini from the following link: