I like to design in the Browser. Sometimes I’m not quite sure if something is fully centered in the Browser window or not, so I made Centerline. Centerline is just a dumb bookmarklet that adds a global ‘center line’ in your Browser viewport. Here it is:

CenterLine

Just drag it onto your bookmark bar, and when you need that center line, click the hell out of it. Check it out:

To remove, just refresh the page.