A quick tip for Gnome users. One of the new features introduced with Gnome 2.4 is the ability to force a window to be always on top. It's a long awaited feature, useful to watch videos while working, checking documentation while coding, etc.
Its implementation can be traced by following Gnome bug #98387, which was opened almost a year ago on November 13, 2002. The bug was not resolved earlier due to a philosophical conflict between the people wanting to keep Metacity's features to a minimum, and the people needing this particular feature. During this bug's life, a few patches were produced, implementing this feature with different ways to control it (a window menu item, a keybinding, etc.).
With Gnome 2.4, it appears that a consensus has been reached with the introduction of the new toggle_above keybinding. If you need this feature, open gconf-editor, navigate to apps/metacity/window_keybindings and assign the new keybinding a key combination.