speedof.me - no Flash
speedtest.net - Flash and non-Flash versions. Full of ads but otherwise a good test site
What is my Browser - shows lots of useful info
Dropbox Referral - Sign-up via this link and we each get 500 MB bonus space
Browser Plug-in Tests
PDF Reader - Loads the printer color PDF file
* These extensions are particularly unsafe. Although the extensions themselves are legitimate, they are frequently found to have security bugs that malware is able to exploit. I strongly recommend uninstalling these. Furthermore, most of these plugins are no longer needed by most people as most websites have moved away from requiring them.