For some time I haev been using iConfactory’s xScope (in demo mode) and I have been really happy with it. But I have also found the Free Ruler for Mac OS X. It is almost as good as xScope, but nowhere near the feature set. Both of these are must have tools for web-designers.
I particularly like this quote from pascal’s website:
Note: Free Ruler is only available for Mac OS X. It may work on Mac OS 9, but this has not been tested. It most definitely will not work on your stupid Windows PC.