InCombatIndicator displays "In combat" if you are in combat in a movable frame. Optionally it can also display "Out of combat" when not in combat, but this is off by default. The frame can be moved, resized and locked.

Type: /ici

for options.

It's meant as a simple, flexible additional way to indicate if you are in combat independent of unit frames, where this information is often displayed by the crossing sword icon (default UI) or some other mean (color for some unit frames).