FlexBar

FlexBar was a powerful actionbar addon developed by Dagar and Thodt that allows the player to place buttons anywhere on the screen. Using an trigger/action structure, FlexBar could also respond to events and hide, show, or move buttons in response.

One of the advantages to this addon was in minimizing unavailable button clutter. For instance, when a warrior switched stances, a good flexbar setup could hide the abilities which were unavailable in that stance.

Note: FlexBar2 is allegedly being worked on.