If you are into analyzing http log files or writing web applications that does user-agent analysis, you might appreciate this website PGTS Agent String Switchword. The website has a huge collection of browser user-agent list ranging from the common Mozilla/Internet Explorer variant to search-engine spider, malicious web bots and internet worms.
The list is organised in various orders to ease up browsing (alphabetical, operating system, by popularity, common robots user agent). Finally, the site also offers tab-delimited user-agent download (zip file). This is useful for those who want to build user-agent database for their own project.