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.
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.