I have used screen rulers for years. What is a screen ruler? A small graphic ruler that sits on your screen and lets you measure things. Sound pretty boring right? When you start using the ruler you find many uses: checking margins, working with photos, web pages, printing, and lots more. Just lining things up in some applications becomes much easier. Many applications have rulers at the edges but being able to put the ruler anywhere on the screen can often save a lot of time.
My favorite ruler was Cool Ruler but I have not been able to get it running on my 64-bit Windows 7 machine and the company that made it, FabSoft.com, no longer lists it on their website. Here is a video showing the rulers I currently use.
JR Screen Ruler is for Windows:
JR Screen Ruler is the ruler I have used the most. It does not let you make tick marks but you can just hang the cursor off the bottom of the ruler and see the measurement. I do find resizing the ruler a little tedious. I have not used the pro version of the tool.
A Ruler for Windows:
A Ruler for Windows allows one tick mark, a center mark, marks for thirds, and even allows you to change the skin. I personally like the classic look of the wooden ruler. Resizing this ruler is a bit tedious as well but not too bad. I did find the buttons a bit small. You can even magnify the screen and the ruler adjusts accordingly. On the whole I like A Ruler for Windows very much.
Working on a Mac? You might take a look at Free Ruler:
I have not used the Macintosh version.
Share your experience with screen rulers in the comments.