ibiblio
Ibiblio: Documents Relating to the Nazi Soviet Nonagression Pact
Public and secret articles of the 1939 Nonaggression Pact between Germany and the Soviet Union are included on this resource. Copies of letters between Germany and USSR concerning the division of Poland, too.
Internet History Sourcebooks Project
Fordham University: Modern History Sourcebook: The Molotov Ribbentrop Pact
This site from the Modern History Sourcebook which is provided for by Fordham University contains the text of the Molotov-Ribbentrop Pact, the Nazi-Soviet Non-Aggression Pact.
Other
The Road Towards the Hitler Stalin Pact and Its Consequences
Essay describing the events surrounding the formation of the German-Soviet Nonaggression Pact in 1939. From a non-English site, but this particular page is English-only.