A programmer in Japan won a Yahoo Japan Problem Solving Engine Award for writing this RealScale browser-based app, which lets you see objects on your screen at actual size. First you enter the model number of whatever monitor or screen you're using (i.e. laptop, external monitor, smartphone, tablet), then it shows you a metric grid at actual size. If you click on sample objects on the list at left, it displays them at actual size.

You can also input dimensions and choose a shape (limited to rectilinear or circular) to display.

The developer's thought was that it could be useful for e-commerce; for instance, imagine you're looking at a case for something and want to see if it's bigger than the item you intend to encase.

One caveat:

If you scroll down the list of sample objects at the site, you'll unfortunately arrive at a comprehensive list of average flaccid and erect penis diameters by country. I'm not kidding, and I'm not sure why the programmer included it.