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