What's the Current Week Number?

This week is ...

Week 21

Week 21 is from Monday May 20, 2013 until (and including) Sunday May 26, 2013.

Week number according to the ISO-8601 standard, weeks starting on Monday. The first week of the year is the week that contains that year's first Thursday (='First 4-day week'). The highest week number in a year is either 52 or 53.

Lists of week numbers by year : 2012 - 2013 - 2014 - 2015 ...

Programming routines

PERL

my $weekNumber = POSIX::strftime("%V", gmtime time);

Replace time with other epoch/UNIX timestamps for other week numbers.

PHP

$weekNumber = date("W");

or date("W", epoch) for other week numbers. Remember to use capital 'W' not 'w'.

Java

Use WEEK_OF_YEAR in the Calendar class.
More info on Stack Overflow

MySQL

SELECT WEEKOFYEAR(NOW())

Replace now() with other dates eg. SELECT WEEKOFYEAR('2008-02-20');
(You can also use the WEEK function with mode=3 select week(now(),3))

MS SQL

SELECT DATEPART( wk, GETDATE() )

Oracle

SELECT to_char(sysdate, 'IW') FROM DUAL

IW: Week of year (1-52 or 1-53) based on the ISO 8601 standard.
WW: Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year (Mostly NOT used)

iPhone/Mac

[NSString stringWithFormat:@"Week %d", 
  [calendar ordinalityOfUnit:NSWeekCalendarUnit inUnit:NSYearCalendarUnit forDate:date]];

Python

import time
week_number_as_string = time.strftime("%W")
week_number = int(week_number_as_string)

Ruby

week_number = Time.now.strftime("%U")

Replace Time.now with Time.local(year,month,day) for other dates.
Formats:
%U - Week number of the year, starting with the first Sunday as the first day of the first week (00..53)
%V - Week number of year according to ISO 8601 (01..53)
%W - Week number of the year, starting with the first Monday as the first day of the first week (00..53)

Microsoft Excel

=WEEKNUM(A1)-1

Week numbers can vary based on your locality settings (not ISO-8601!). See discussion below.

Linux/Unix

date +%V

Returns the ISO 8601 week number.
Other formats under 'Ruby'. More details on Kernel.org

PowerShell

Get-Date -UFormat %V

X++ (Microsoft Dynamics AX)

int weeknum;
weeknum = weekOfYear(today());

Thanks to everyone who sent me corrections and updates!
Also see: Current day number