|Gilder Lehrman Institute of American History: In a word: Spectacular. The University of Houston has come up with one of the best sites on the Internet for locating information on American History. Newspaper articles, maps, photos, film history! Clearly, these guys beat the folks at Vikingship (that's us) to the punch. Maybe a bit short on the side of biographical information, but certainly well organized and the UH people are clearly using very reputable links for their information, from the NY Times to the LC's "American Memory," the "Gilder" site is an impressive show of American historical punch.
Encyclopedia Smithsonian: Although not history specific, the site, coming from one of the most renowned national museums in the world, provides excellent and reputable coverage of a number of historical events. In addition to readable passages, the site will also provide a list of suggested readings in the area of note.
Catholic Encyclopedia: The Catholic Encyclopedia has long been a primary feature of reference collections in all libraries. Now also residing online, this thorough and even-handed source is particularly strong in the areas of church and diplomatic history. It will also provide solid biographical information on a variance of historical figures. With other, more general sources available (see above and below), this should not be considered a source of first resort, rather one that should be considered supplemental.
Andromeda History Encyclopedia: This is one of the many great features on the History Today Web site. As a scholarly publication which has been around since 1951, one will find the site reliable and quite large in scope. The alphabet-based index is easy to use, and the entries are clear and lucid. As a drawback, the site lacks depth in its entries, most running no more than a few sentences in length. In short, "Andomeda" is a convenient spot to stop for quick-hitting ready reference info.
Information Please History & Government Encyclopedia: Information Please has always been known for its great timelines, and this time is no exception. A major drawback of the site is its big slant toward U.S. History. What's more, its organization is a bit lacking. Nonetheless, IP will deliver good, reliable information on some pretty general topic areas.
World History: HyperHistory: I don't know a lot about these guys, but count this as a clever site that brings three distinctive style concepts together: Text, Timelines, and Maps. Fused together via hyperlinks, the site fashions nice, clear graphics and color-coding, while keeping nearby interpretive text. Drawbacks include the lack of more text, summaries, explantions of particular events. In sum, however, a really stellar site showing hard work and ingenuity.
Encyclopedia.com from the Electric Library: An information rich, general encyclopedia. Jump off point is a browesable alphabetical index. Subject entries are free, lengthy, and very helpful. Articles are available for a price (unlimited access for $14.95 a month), although a seven day trial is available. Pictures and maps also fall into the fee-based category, but the basic service of providing full and helpful entries should be enough for most of us.
Internet History Sourcebooks Project: A great site maintained by Fordham University prof. Paul Halsall. This is primarily a metasite; in essence, one made up of other sites. Scope is tremendously large and the site is made up of the following sourcebooks: Ancient History, Medieval, Modern History, Byzantine Studies, African, East Asian, Global, Indian, Islamic, Jewish, Lesbian and Gay, Science, & Women's. Readership is geared more toward older, academic-centered learners.