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.
Wikimedia
Wikipedia: Molotov Ribbentrop Pact
An information-packed encyclopedia article from Wikipedia on the Molotov-Ribbentrop Pact presents a good summary of the events that led up to the signing of the pact. There is also a link to the text of the pact.
The History Place
The History Place: The Triumph of Hitler: The Nazi Soviet Pact
Describes the alliances that were forming between countries in the lead-up to the outbreak of World War II, the negotiations between Hitler and Mussolini, and the Nazi-Soviet Non-Aggression Pact signed by Germany and Russia.
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.
The History Place
The History Place: The Nazi Soviet Pact
This site from The History Place provides a photo and caption of the signing of the Nazi-Soviet Nonaggression Pact. Includes a text copy of the agreement, with links to additional information.
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.