Hi jrb599,
I am a rabid HAL (Holland America Line) fan so I have not had the pleasure of trying the other cruise lines yet.
I am in the Pacific NW so I sail out of Seattle. The HAL ships out of Seattle tend to go to Glacier Bay or the Hubbard Glacier on there 7 day round trip cruises. They stop at Juneau, Sitka, Ketchikan and Victoria BC. I have found that the HAL ships leaving from Vancouver BC go to Skagway instead of Sitka but also stop at Juneau, Ketchikan and Victoria BC. I haven't been to Skagway yet but I am sure it's a great city to explore. I have been to the others mention above twice now and I have always found something new to see every time I go back.
If you haven't decided or book your cruise yet , may I suggest talking to your local travel agent He or she should have all the cruise line books going to Alaska.
Also stop at your local book store and check out the travel section / Alaska / look for Fodor's " Alaska Ports of Call 2008 " I use this book as a guide and it comes out every year with updates. I believe that Frommer's also has that same type of book. Alaska is very popular.
Don't wait to long to book, these cruises fill up fast. Good luck on your cruise. Your in for a lot of fun and great sites.