People frequently ask about the relationship between the GNOME project and window managers. GNOME does not specify a particular window manager. Although the default is Metacity, it is intended that any window manager can be used. The reason for this decision is that many people are attached to their particular window manager; forcing them to switch just to use GNOME would be counterproductive.
However, to work well with GNOME, a window manager must provide certain features which some window managers do not implement.
A GNOME-compliant window manager should implement the MWM extended window manager hints. Some GNOME applications will use these hints to increase usability, as outlined in this window management standards proposal.
KDE and GNOME have been working to increase interoperability between the two desktops, and have developed a Window Manager Specification that can be found on freedesktop.org. It describes some extended window manager hints that all window managers ought to support.
Last, a GNOME window manager should also be a client of the session manager, following the X Session Manager Protocol. This is a requirement for a window manager to be considered even minimally GNOME-compliant.
If you are a window manager author and would like to join the discussion of the extensions and additions, please join the wm-spec-list@gnome.org.
At the moment there are a few GTK+-based window managers available implementing those specs. Here are two of them:
As of version 2.0, Metacity is the default GNOME window manager. It is well-integrated into GNOME and features decent implemented basic window manager capabilities defined by the specs mentioned above. If you're an average user, this is definitly your choice.
In former times Sawfish was GNOME's default window manager, but it was too complex to maintain. However, some power users prefer its extensive feature set and the ability to extend the window manager using Lisp.