Do you find yourself constantly using the 'Other...' item in Safari's Develop » User Agent menu to simulate User Agents that aren't shown in the list? If so, it's easy to permanently add your own favorites by editing the UserAgents.plist file in the Resources directory of the bundle.
I recommend making a copy of Safari.app, then Control-click your copy and select Show Package Contents from the pop-up menu. Navigate into Resources and edit the file UserAgents.plist in any decent text editor to include your faves.
Simple, yet effective.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090625002236616