Unfortunately, Voyager has not been updated for Mac OS X, so the following only really applies if you are running an older system. Starry Night is an excellent program for X and Equinox is a reasonable shareware astronomy program.
Voyager is a well-designed, complete, fast astronomy tool. It is a program that any amateur astronomer can use to plan an observing session or just to explore the sky. It is also the most useful piece of software I have ever seen for teaching astronomy. For example, imagine being able to teach the geometry of eclipses by observing the same eclipse from different points on the Earth, from the Moon and even from the Sun!
Depending on your requirements it can fit compactly on a hard disk without consuming too much space, this makes it practical to run on a Powerbook and use in the field. A full installation of all the star catalogs and graphics takes a lot more space, of course.
Like all the programs I advocate on these pages I have no connection with the manufacturers and recommend these products solely on how useful I found them for work done for my clients and myself.