Google Maps now shows you the distance of any line you draw on a 'My Map'.

http://blog.radioactiveyak.com/2007/09/distances-on-google-maps.html

They didn't invent this, Yahoo! has done it for a while, and I know of a couple of mashups that do the same thing, but still. It's pretty handy!

Google planimeter measure areas too
www.acme.com/planimeter/

"They didn't invent this, Yahoo! has done it for a while"
or maybe it's from google earth, and yahoo copied them. ;)