How about when Dax was given command of the Defiant, while Sisko was doing his strategic stuff at the starbase. Not captain rank, but served as captain.
Same thing for Data, aboard the Sutherland.
Worf was captain very briefly aboard the Enterprise-D, twice - it is debatable whether it was official either time or not, though.
Nog was shown as captain of the Defiant in the future, and T'Pol was captain of the NX-01 in a future that did not come to pass.
The Doctor, acting as
ECH, was captain of Voyager on a few occasions.
