ubuntu has this sort of thing on by default, but when i used osx all day I wanted the current date marked in cal, and golfed the hell out of it. I can't think of anything shorter than:

cal | sed "s/.*/ & /;s/ $(date +%e) / [] /"

which gets you a nice:

   November 2008     
Su Mo Tu We Th Fr Sa 
                  [] 
 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