Here are some ABAP reports rarely used bit helpful for ABAPer:

RSCLTCOP
Copy tables across clients
RSAVGL00
Table adjustment across clients
RSINCL00
Extended program list
RSBDCSUB
Release batch-input sessions automaticly
RSTXSCRP
Transport SAPscript files across systems
RSORAREL
Get the Oracle Release
RGUGBR00
Substitution/Validation utility
RSPARAM
Display all instance parameters
RSUSR003
Check the passwords of users SAP* and DDIC in all clients
RSUSR006
List users last login