I'm busier than I want to be at the moment. Anyway, via James:

$ history|awk '{a[$2]++} END{for(i in a){printf "%5d\t%s\n",a[i],i}}'|sort -rn|head
  255   hg
   54   ls
   42   cd
   19   less
   18   python
   14   vim
   11   bzflag
    7   screen
    7   bzr
    6   ./run_tests.py