The Library of Congress has digitized vast collections of manuscripts, photographs, audiotapes, films, and other resources, and they are putting more material online every day.
Chronicling America is an initiative between National Endowment for Humanities and the Library of Congress. It makes numerous newspapers available and searchable back to the 1800s. This is a great tool for researchers.
The Texas State Historical Association offers the Lone Star History Links, an exhaustive online bibliography of links to several thousand Texas history primary source documents located in many online collections.
National History Day shares an extensive list of resources for historical research. Also available on the NHD website is guidance on how to design a project, how to develop a topic, and a great deal of other helpful information.
Texas History Day is sponsored by the Texas State Historical Association. They provide many educational offerings, including History Day, Junior Historians, and History Awareness Workshops. TSHA also offers a vast amount of information on Texas history through the Handbook of Texas Online.