kati@hogwarts:~$  history | awk '{a[$2]++ } \
END{for(i in a){print a[i] " " i}}' \
| sort -rn | head

36 cd
29 sudo
18 ls
13 apt-cache
9 python2.4
5 bin/buildout
4 mc
3 paster
3 less
2 touch

[via Nick Ellery]