I wrote this little Excel hack for adding back leading zeros.  This comes in handy when someone sends you an Excel file and you notice that some of the fields are missing leading zeros.  SSNs and zip codes tend to be common culprits.

Fix SSNs - 9 Digits

= right$("0000000000" & [A1],9)

Fix Zip Codes - 5 Digits 

= right$("00000" & [A1],5)

-Chris