Cap Holes
Cap holes closes gaps between faces by inserting polygons, in effect removing "holes" from a mesh. It can also operate on selected faces.

The above is a cube that was converted to an editable mesh. The cube on the left has a polygon face removed, the one on the right is the same but with the cap holes modifier applied. The new face is a quad (four sided polygon) but the cap holes modifier has an option to allow two triangles to be used instead by checking triangulate cap.
This modifier works best on planer surfaces, but can return reasonable results on non-planer surfaces such as spheres.
