There are several museums of science which include slide rules as part of their collections.

Museum Notes
The Whipple Museum of The History of Science, Cambridge UK This museum has a collection of around 4 500 scientific instruments which includes around 100 slide rules. At the moment only about 10 of the rules are on display but by the end of 2000 most of them should on display. The site also has links to other science museum sites
The Science Museum, London, UK This museum is the UK's premier science museum. Its collection includes a Bissaker rule  of 1654, believed to be the earliest straight slide rule in existence.
The MIT Museum The MIT museum holds courses on how to use a slide rule which the students seem to enjoy.
The HP (Hewlett Packard) Museum. This museum has a section on-line devoted to slide rules.
Humboldt State University Scientific Instrument Museum This museum appears only to have one slide rule, a Keuffel and Esser demonstration rule.
The Macleay Museum of the History of Science, University of Sydney, Australia This museum has some slide rules in its collection but no on-line images.