Screen Rulers! (native for MacOS X!)
FreeRuler looks to be just like the Screen Ruler that I’ve been using for years (on MacOS 7,8,9 and Windows 95, 98, NT, 2000) , but now available from a new author, for OSX. Why do you want a ruler on your screen? I’ll tell you why – so you can see how big a graphic on a webpage is — so you can see how wide a layout is, so you can size graphics to fit in that area appropriately. It’s a super useful thing if you ever ever do anything visual. And even the programmiest of web programmers occasionally needs to do visual stuff. If you’re a designer, you need this, like yesterday. I think I’ve been using the original Screen Ruler since 1996 or 1997. Here’s the author with it in his .sig on usenet in 1995. And in case you hadn’t noticed, that’s over 6 years ago. Eons in web time.
It’s a classic and necessary web development tool.