Growing out of the shell that was cc:Betty, Threadbox aims to do the same kind of multi-recipient email organising, but with a more business/client focus. To your email conversations, Threadbox attaches tasks, files, maps and other collaboration data.
You can see Threadbox in action in the clip above, and in a little two-person example above. Threadbox sits in a few different categories, serving as a kind of project organiser, a Google-Wave-like conversation threader and attachment handler. Its best feature may be that, for clients and coworkers who don't love webapp sign-ins, your threads can be managed by email replies.
Threadbox is free while in public beta.