Kovacs Consulting, which holds online training for librarians, also surveys librarians' core reference tools in a variety of subject areas. And yes, there is a Government Documents section! You can contribute to the current survey or take a look at the 2005 and 2006 results.

What a great way to find some new resources, and see what everyone else is using! I also plan on using the list as I train a graduate student on our reference desk.