QSL Routes 

IK3QAR
K4UTE QSL Manager Database (no longer being updated as of October 31, 2006)
OZ7C QSL Manager Database
Pathfinder
QRZ QSL Manager
QSL Managers Society
SM5ARL QSL Address Database (has not been updated since August 11, 2007)
SM5ARL QSL Manager Database
The Daily DX - QSL Manager Listing
WWW.QSLINFO.DE - the Internet's #1 QSL information search!

Frank's Compulsive Guide to Postal Addresses - Effective Addressing for International Mail (excellent resource for direct QSLing)

IRCs - What International Reply Coupons are valid.

QSL Collection - Great history and much, much more

IRCs or Greenstamps? (W9OL)
Another IRC and Greenstamp chart (N6DHZ)
Another Greenstamp chart (K4HB)
Rückporto für QSL-Post nach Deutschland (DL6ZFG)

Currency converters
Oanda.Com
Postal Rates.Info
Postage Rates Worldwide