If you live on week cycles, it is quite nice to see the week number in your calendar. It is not (yet) provided as standard feature in the user settings for my calendar solution of choice.
It works, but ... However I admire the power and possibilities of the user-scripts, it is fairly heavy tool for such simple thing. My Firefox install is pretty large thanks to variety of developer-oriented plugins, and I do not want to slow it down even more. Another issue with scripts is potential security implication - recently (as you will see on the script site) a malicious user started to upload scripts stealing cookies. What this means for
slightly paranoid security-aware person like me, is to read and understand the source - and that is way too much trouble.
Fortunately, there is better solution using Google API's - you just subscribe to a calendar that has the information :-). Simple, elegant and as secure as the calendar itself :-) - all you have to do is to click on this link (for weeks starting on Sunday).
The page for the non-greasy solution is http://recover89.googlepages.com/googlecalendarweeknumbers - it contains link for Monday based weeks as well.