Inspired by Simon, I thought I'd follow the crowd;

andy47@Mort:~$ history|awk '{a[$2]++} END{for(i in a){printf "%5dt%sn",a[i],i}}'|sort -rn|head

  187   cd

   66   whoswho

   46   svn

   30   python

   20   rm

   16   ls

   15   open

   13   vi

   13   grep

   10   ipython