One of my favorite tools when I was at school was the good old ruler, athough I found myself losing them all the time.
If only I'd had Pocket Ruler on my mobile phone (which weren't invented when I was at school, incidentally) I wouldn't have had this problem.
This simple application adds a ruler to your Nokia display, allowing you to take measurements quickly and easily.
The program displays measurements in milimeters and centimeters and even has inches displayed on the other side of the ruler.
One of the best things about the app is the fact that the ruler display can be rotated, so you can use it either vertically or horizontally.
You'll need Flash in order to use Pocket Ruler, but it's dead easy to install and while it doesn't really do much it's highly functional for taking measurements of small things.
Of course, if you want to measure-up larger objects things become a little more difficult though.
If you're fed up of losing your ruler all the time, Pocket Ruler makes for a handy replacement.