Arriving in Boston Friday, May 17th for a week. During this time we would like to take some bus tours to Salem, Cape Cod and Plymouth. Gray Line tours do not begin until after Memorial weekend. Are there any other bus tour companies I could contact?

How about renting a car? That probably would be cheaper.

Do you have any particular reason for wanting to take tours, rather than renting a car and driving yourself? The destinations you name are easily accessible on major highways (you even go through Plymouth on the way to Cape Cod). They are also accessible via public transportation -- trains and/or regular buses.

Assuming you do not want to drive, you can easily take a commuter train from Boston to Salem via North Station. Once in Salem, there are plenty of sights to see within walking distance of each other, and you'll have more time to explore if you're not tied to a bus tour. Be sure to see the Peabody-Essex museum, the House of Seven Gables, the National Park Service visitor center for information on all the above. Some of the witch museums are hoaky and a waste of time/money, but one is supposed to be okay (don't know which one). Just take the train and enjoy the visit.

Ann's recs are great. The one "witch" museum that rather captures the history in a dramatic way that I would recommend is the "Witch Museum." Sort of a "still life" panorama of "The Crucible."
The Witch House is a big bore - just some judge's old house full of drab antiques. The House of Seven Gables is much better and in a lovely location. Other "witch draws" have local actors doing hokey reenactments.