The links are pretty good too for postal rates. Particularly easy to read. Good to bookmark.

This set is very good for domestic US:
http://www.usps.com/consumers/domestic.htm#H4

This is the same, but you don't have to click links to get to some tables. All the tables you would ever want seem to be in this one long page (for domestic US):
http://pe.usps.com/text/dmm300/ratesandfees.htm#wp3804548

This is good old Publication 51 for International rates from the US:
http://pe.usps.gov/text/pub51/welcome.htm

Robert