I was looking for a way to demo themes the other day and found that most theme switch plugins were largely out of date and didn’t work correctly with current versions of WordPress. The one that worked the best was Theme Switcher which added a widget to allow switching. The problem with this was that not all themes call their widget areas the same. Switch to a different theme and the widget disappears leaving you with no way to switch to another theme.
With the advent of the admin bar in WordPress 3.0, it seemed like a logical place to put it and it will work across all aspects of the site. This plugin adds a new drop-down called Theme Switch that gives you a list of all installed themes.
You can grab a copy from the WordPress plugin repo or install from Add New Plugins (Search for “admin bar theme switcher”).