Do you always find yourself constantly using the 'Other' item in Safari's Develop User Agent menu to simulate User Agents that just won't be shown in the list? If so it's not that hard to add your own favorites by changing the UserAgents.plist file in the Resources directory of the bundle.
you really need to making a copy of Safari.app, then right click your copy then select Show Package Contents from the pop up menu. Navigate in to Resources and change the file UserAgents.plist in your text edit/text wrangler to include your faves.
very handy.