Pressing the Shift key twice activates the SharePoint Workspace hotkey. If you have unread messages or invitations, the hotkey ...

Pressing the Shift key twice activates the SharePoint Workspace hotkey.

If you have unread messages or invitations, the hotkey opens the next item for you to read.  Otherwise, the hotkey opens a window for you to send a new message.

The hotkey works even when SharePoint Workspace is not the active application or is running only as an icon in the system tray.