Enco has a relatively small milling machine table vise for $33.96. It may not be the world’s finest, but it does help you out in a pinch and makes projects that were a bear to do before a little easier. It’s also pretty easy to find a home for it when not in use.
Unlike a regular vise it mounts to the table at either end of the work, which allows pieces to be held that are much longer than a usual vise can manage. The length of the two vise halves is 11″, so on a 30″ table you would be able to clamp a 19″ long workpiece.
Milling Machine Table Vise [useenco.com]