An interesting idea for a web-based email site. I don't think I've seen one (not that it really makes any sense, but I can see how some people would like it). Closest you have are drafts, which are pretty darned close, but it's not quite the same, no.
I still fall on the "don't send it unless you mean to send it" side, however. My outbox is only used when I don't have internet access.
Offline Gmail certainly has an Outbox. The main reason being you'll generally have no internet connection or a very flaky one. By product of it is that yu can indeed cancel emails after sending them.
It wouldn't be a bad idea to use an Outbox for some people. Gmail could offer it as a labs feature and whenever you request or end your session/logout it would send your email...
Removing email from an account once sent like above seems like a very bad user experience quirk.
I still fall on the "don't send it unless you mean to send it" side, however. My outbox is only used when I don't have internet access.