Microsoft also is believed to be working on a lightweight collaboration and document-sharing app (which may be using "Flip" as its working name), as well as a lightweight project-management application, (which may be known as "Highlander"), according to sources of mine who asked not to be named.
Flow, Flip and Highlander are all productivity apps that are aimed at mobile users. My bet is they're all the handy work of the "Do More Experiences" team that is part of Microsoft's Applications & Services group, as that team is focused on redefining productivity and building "next-generation experiences" for mobile platforms, including phones and tablets.
One of my contacts said Flip may include some of the features that will be part of the Flow email/chat application, but will go beyond Flow by offering document viewing, editing and collaboration features.
Highlander is meant to provide lightweight project management for smaller-sized projects, making it easy for users to update tasks and watch projects' progress, that same contact said. Microsoft already sells a more fully featured project-management application, Microsoft Project.
Flow and/or Flip might eventually integrate information from sources such as LinkedIn and Facebook for Work, allowing users to keep tabs on conversations and email messages from "people you want to listen to," my contact said.
Flow and Flip are not next-generation versions of Yammer, Microsoft's enterprise social-networking app, from what I'm hearing. Microsoft is taking a "new product direction" with Yammer, but it seems the plan is to continue to offer Yammer as a standalone social-enterprise app, at least according to a Microsoft blog post from earlier this week. (Microsoft does plan to disclose more about Yammer's future direction during a YamJam on May 27, the blog post says.)
Office 365 isn't a requirement for Flip or Flow, either, my source says. Flow does use Exchange on the back-end, my sources say -- and Neowin's Brad Sams' do, as well. (Sams has some screen shots of what Flow may look like.)