This is added by "SideBarEnhancements" package
F12 key allows you to open the current file in browser.
If you want to add a url to that feature: Right click the sidebar -> project -> edit project
There add the new preferences:
"url" allows you to set the url of your local server, opened via F12
"url_production" allows you to set the url of your production server, opened via ALT+F12
an example of my project file
"path": "/C/Documents and Settings/tito/Datos de programa/Sublime Text 2/Packages"
This will only work with the latest version, update and be sure to restart sublime.
There is also other packages which add a more convenient "browser preview", for when you needing to check a lot the result.
[edited] for clarification