Not exactly what Aaron Swartz wants, but pretty close: you can use GMail via POP with any email client. I use it that way with Eudora, so that I have offline access.

A free Gmail clone. A lot of people I know use Gmail for email. It's not because they don't have access to servers or can't afford a couple gigabytes of disk space. It's because Gmail is simply the best interface for email out there right now. It'd be even better if it was free software, though.

The biggest problem with Gmail is that you can't run it offline. But if it was free software, you could run it on your local machine and use it even when not connected to the net. This would also have the nice benefit of making it much faster for the user. Some synchronization code would be necessary, but it'd be worth it.

For me, GMail works like any other email system that way, except that it provides easily searchable archives to boot