Here you can find a collection of undocumented (not on Cisco Dokumentation CD or in online help) IOS commands:
csim
show interfacetching
show ip ospf statistics
debug ip ospf monitor
ttcp
if-con
no snmp-server sparse-tables
who
show region
ps -c
quit
test aaa group radius
list of more and more undocumented commands
If you know more undocumented IOS commands for this collection, please send me a e-mail.