cal command - start monday
GNU cal has a --starting-day option:

$ brew install gcal
$ alias cal='gcal --starting-day=1'
$ cal

      June 2013
 Mo Tu We Th Fr Sa Su
                 1  2
  3  4  5  6  7  8  9
 10 11 12 13 14 15 16
 17 18 19 20 21 22 23
 24 25 26 27 28 29 30


ncal also starts weeks on mondays by default:

$ ncal
    June 2013
Mo     3 10 17 24
Tu     4 11 18 25
We     5 12 19 26
Th     6 13 20 27
Fr     7 14 21 28
Sa  1  8 15 22 29
Su  2  9 16 23 30