This is for all of those who have gotten caught out there with a printed piece that couldn't go through the mail and the designers who stay awake at night wondering if they left enough room for the barcode.
Postal Explorer - pe.usps.gov/
Ribbs - ribbs.usps.gov/
Zip4 lookup - zip4.usps.com/zip4/welcome.jsp
Below are downloadable PDFs of common sizes of pieces that you might need to mail. Complete with "off limits" areas, designated return address and indicia areas and sizes to accommodate bleeds.