BBC Bitesize - Long-term Causes of the Russian Revolution - A easy to read introduction to the causes of the Russian Revolution. Perfect as a jumping off point to understand the basics before delving in a little deeper.
The Marxist Internet Archive - An all-volunteer, non-profit, public library that contains the writings of 592 authors in 45 different languages. A good place to understand the role of Marxism in the Russian Revolution.
The BBC - War and Revolution in Russia - An online study guide that outlines the causes of the revolution and provides a detailed timeline of events. Includes links to other websites, has images, and has several video documentaries.
Russian Revolution - Primary Source documents from the Internet History Sourcebook compiled by Fordham University