According to man cal, there's also an ncal command with extra options, such as displaying week numbers via the -w flag. But no ncal is installed in OS X. A little experiment reveals, however, that it's one and the same program -- it apparently behaves differently when named differently.
If you create an ncal link to the cal utility, you get a fully functional ncal program; note the week numbers below the calendar:
$ mkdir -p $HOME/bin
$ ln `which cal` $HOME/bin/ncal
$ $HOME/bin/ncal -w
May 2008
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22 29
Fr 2 9 16 23 30
Sa 3 10 17 24 31
Su 4 11 18 25
18 19 20 21 22
[robg adds: This works as described; you can also now use ncal -e or ncal -o to see the date for Easter (Western and Orthodox, respectively), -J for the Julian calendar, and -p to see ...
