Search thousands of primary sources from the National Archives. This is probably the most comprehensive primary source collection you can get online. You can browse by source type, time period, and recommended sources. You should definitely use this site, no matter what your topic is.

The Library of Congress is one of the largest collections of primary source material anywhere. The LOC collects and archives millions of documents, photographs, letters, speeches, audio and video files.

Search through the Smithsonian’s extensive collection of primary sources: artifacts, documents, and photographs. The collection can be broken down by topic, so you can browse or search to find useful things.

This site is a U.S. history survey course, and it does provide links to assignments and classroom tools, but you will probably find it most useful for its database of primary sources. There are a lot of links on this site, so it's easy to fall into a history rabbit hole, but if you're looking for a specific document, (for example, a letter or speech quoted somewhere else) you can often find it here.

American Heritage is a magazine about history, with an enormous amount of information available online. The articles are easy to understand and don’t require a lot of background knowledge. In addition to timelines and biographies, it has critical thought and academic writing about history.

Like American Heritage, but better! HistoryNet is a collection of many history magazines, whose archives you can search.

HERB is a search engine from the City University of New York's history department that let’s you browse primary sources by time period or theme. It focuses on social history, so how real people lived and how the events of history affected them.

Calisphere is a project where lots of libraries in California (public, university libraries, and fancy specialized private libraries) put all of their historical information together online. There are documents, images, recordings, and more. You can search by topic in the search box, or click on "Exhibitions" or "Collections" to see specific topics or specific libraries.

This collection of primary sources connected to women’s history is incredibly comprehensive. Each decade contains hundreds of different primary sources: documents, artifacts, photographs, letters, etc. You can search by time period or by using a specific search term.

Studs Terkel is a prominent oral historian, which means that he has basically been interviewing people about historical events for several decades now. This website contains audio files of the vast majority of Terkel’s interviews, which you can search through. Obviously, this is mostly going to concern the later parts of American history - the history that happened to people who were alive from 1952 - 1998. He tends to interview non-famous people, so you might not have any luck searching for specific people, but you can search for things like "dustbowl" or "farms" or "new deal" or "depression".

Archiving Early America is another collection of primary source material, with links to other resources, plenty of maps, and an especially large gallery of images from the time period. There are a lot of ads for the tutoring company who bought the site a couple of years ago, so it’s not the best looking, but it is worth a visit for the images – some of them can’t be found on other websites.

This won’t be useful for every topic, but Picturing America is a collection of famous American works of art (works of art that depict elements of American life, notable Americans, or American landscapes). Each work has some background information about it, and (even better) a list of related sources that bring you in depth information about the art and also its subject.

For the most part, this website is a companion the book America: A Narrative History (which you have to buy); however, they have one really neat feature you can check out: downloadable Google Earth tours of different historical themes. So, you can use Google Earth to see a geographical overview of the Revolutionary War. Each stop on the tour has some information, and also links to multimedia resources and places to get further information. You have to use a VPN for the really cool Google Earth stuff, obviously - but it's just too awesome to leave off the list.

This may not be applicable to every topic. The speeches are arranged by speechmaker, but you can also easily search by topic. Each speech page includes the text of the speech, background information about the speech, and, in some cases, audio.