The United States Exploring Expedition, 1838-1842 is a Smithsonian Institution Libraries Digital Collection of interest to historians of the northwest. (You can see additional digital collections here.)
The four-year expedition to explore and document the Pacific Ocean was by far the grandest (and most expensive) scientific endeavor undertaken by the United States government before the Civil War. This map of the expedition's route gives a sense of the scale of the project. Another way to explore the expedition geographically is to look at the "About this Book" page on Google Books Search to see a map of all the place names mentioned:
But back to the Smithsonian digital exhibit. The site has the full text of the expedition narrative and all the scientific reports, as one would expect. There are also a series of very good essays, commissioned for the project, led off by an excerpt from Nathaniel Philbrick's book Sea of Glory: America's Voyage of Discovery - The U.S. Exploring Expedition, 1838-1842. There are also some excellent supplemental materials, including rosters of the crew of each vessel and a very sophisticated searchable database of "Cultural Artifacts Collected During the U.S. Exploring Expedition." The wonderful illustrations from the reports are in another searchable database and available in extremely high quality downloads. The illustration below is of "A Kalapaya lad. A native of Oregon."
This website is a nice example of a focused digital presentation that takes a very well-know set of primary sources and makes them searchable and useable in new ways, while linking the sources to other resources such as the cultural artifacts database.