There are two main cloud IDEs currently available:
Both of these offer zero-installation, completely in the cloud development. Combine these with a package like Cloudbees or Heroku, and all you need is a web browser to build entire applications. Not just toy apps, either - real, full applications.
There are a few scenarios that are particularly attractive about a cloud-based IDE. For example, we do a lot of functional web testing. Having an entirely cloud-based infrastructure (combining Sauce Labs, for example), would let a test team work very fast - no need to worry about going back and forth with IT to set up IDEs, jobs, or servers - just click and go. The sharing features built in to the cloud-based IDEs are particularly nice for collaborative development (e.g. sharing work between the devs and the SDETs).
What do you think? Have you used a cloud-based IDE? Would you be willing to give up your normal IDE and switch (or even try) a cloud-based package?