Check out Ziteboard – an online whiteboard with real-time collaboration running on any device with any OS. It is a web app, means it uses pure html5 + javascript, so it is cross-platform software. The UI is minimal, the working space is maximal.
Offline: an app can't be offline and collaborative in the same time. However Ziteboard works well if no internet connection. Collaborators can draw offline and all drawings will be synced after reconnection.
Self-host: Ziteboard doesn't offer on-premise solution, but something close: true white label solution. The app can be customized to your brand and needs. The service runs under your domain and can be embedded into your page, web app.