Week numbersCalenGoo can display week numbers in its day, week, month, agenda and year view. You can configure it separately for each view under "Settings > Display and Use > Day/Week/Month/Agenda/Year view > Week numbers".
Week numbers in Google CalendarIn Google Calendar you can view week numbers by subscribing the "Week numbers" calendar. Just sign in to Google Calendar on your computer, click the gear icon in the upper right corner and then "Settings > Calendars > Browse Interesting calendars > More". There you can click on "Subscribe":
Afterward the week numbers are displayed as small numbers at the top of the calendar: