The Library of Congress has an enormous collection of primary sources on a range of topics, most related to United States history, but they have some global collections too. Consider browsing their digital collections to see if you can find a topic that interests you.
The Digital Public Library of America gives you access to over 29 million images, texts, videos and sounds across the United States. You can browse by topic, check out online exhibitions, or check out their primary source sets.
University of California San Diego has a great guide full of links to digital archives and other primary sources on a range of topics.
DOCSTeach - a collection of primary source documents from the National Archives. You can filter by specific era, limit by particular type of source and also search for specific items.
Hurricane Digital Memory Bank - a collection of documents, videos, photos and other items related to the Hurricanes Katrina and Rita.
Internet History Sourcebook - created by Fordham University this collection focuses on America as World Leader: Internal Change.
September 11th Digital Archive - includes emails, first hand stories, photos and videos surrounding the events of the 9/11 terrorist attacks.