At one point I found myself composing some email (started with notes for a conference, then code-heavy email) in Github's editor, rendering, and then pasting into an email. And then I realized that I'm a programmer and can make stuff to do stuff. So I made Markdown Here (or the project page on Github). It's a Chrome/Firefox/Safari/Opera/Thunderbird/Postbox extension that allows you to write email in your normal email editor, but in Markdown, and then render it before sending. It also supports syntax highlighting and some TeX math support. And it works in Google Groups posts, Evernote (web interface) notes, Blogger posts, and so on -- anywhere that uses contentEditable or designMode for the editor (including TinyMCE-based and CKEditor-based ones).

So, yeah, I use Markdown for email. There are about 15,000 Markdown Here users, so I guess they're part of the answer too.

I'm one of the founders/developers of a product that's an alternative to Groove -- CipherShare. We're a very small company, but the product has been around for about 7 years now, and we have some pretty sizable customers (Bechtel, MDS Sciex).

We have: a very clear, simple interface; zero sensitive data exposure on server (we have a reseller who will host for you, if need be, and won't be able to see your data); support for very large files; secure chat; optional account/password recovery; file-type-agnostic document handling; auto-delta-versioning; etcetera. Check out the site and email us if you'd like a demo (we can host it for you, or you can host it yourself).