10/26/11 23:02:38 ¶ ●

Add week numbers to iCal and Google Calendar using a shared iCal calendar

Getting the aforementioned week number subscription working without Google Calendar is a bit more tricky, as there is no special site for it, but luckily iCalShare is still maintained ad online.
First search iCalShare (click link) and search for the calendar in your timezone. Then subscribe (Calendar → Subscribe…) to get week numbers showing up in iCal.