go to settings > devices > keyboard
look for the keyboard shortcut for "Switch windows"
set this to the shortcut Alt+Tab (this will overwrite the old shortcut)
If you now press Alt+Tab you will be able to directly select all open windows without grouping into the different apps.
Come April 26th and the release of Ubuntu 12.04 a vast number of Ubuntu users will be getting their first taste of the Unity desktop since its basic beginnings as the Ubuntu Netbook Remix in the last LTS. But for all of the innovation, aesthetics and features that the unity desktop offers there is a chance that, for many, the change will be too much. But Unity is only one desktop environment available to install in Ubuntu.
This section discusses helpful tips for getting GDM working. In general, if you have a problem using GDM, you can submit a bug to the "gdm" category in bugzilla.gnome.org or send an email to the<gdm-list@gnome.org