PBS's always-impressive American Experience documentary series created a fine website to accompany its 2006 episode, The Gold Rush. The website is loaded with primary sources and archival images, and includes a full transcript of the television program, which features interviews with several of America's best practicing historians.
As part of the 1998 sesquicentennial celebration of the discovery at Sutter's Mill, the Oakland Museum of California created a major online exhibit. Don't miss the very strong emphasis on the art of the Gold Rush.
The California State Library hosts the online exhibition, "California As We Saw It": Exploring the California Gold Rush. As we might expect from the state's leading historical archive, the exhibit is comprised almost entirely of primary sources. Potentially a great first stop for writing an original paper on the Gold Rush.