Encyclopedias
Encyclopedias were, early in the days of the Web, all the rage. There are not as many of the general encyclopedias online now, and many of those available cost money. But there are still some fairly good free versions available.
Britannica—http://www.britannica.com/
The Encyclopedia Britannica site is not your grandfather's 35-volume, drop-it-on-your-toe-and-deeply-regret-it encyclopedia. You can search many different kinds of information here (encyclopedia information, Web sites, multimedia, and so on). The snag is that there's not a whole lot of encyclopedia content that's free. If you search for something, you'll get an excerpt of the first part of the encyclopedia article, but not more than that. If you don't mind shelling out some money, subscriptions to give you complete encyclopedia access are $9.95 a month or $60 a year. Free trials are available.
Encyclopedia.com—http://www.encyclopedia.com/
Encyclopedia.com is a service of eLibrary, which in turn is a pay service. Unlike Encyclopedia Britannica, however, you can get the full text of encyclopedia articles from this site; they're just fairly short articles. From the front page you can view some “Today in History” information, browse the encyclopedia alphabetically, or search by keyword. Articles as I mentioned are fairly short, with no multimedia or additional information. Each article also has recommended resources from the eLibrary online database, which does cost money to access.
TIP
Want to try an encyclopedia that's completely free and has extensive articles? There's one snag, though: it's pretty old. Almost a hundred years old, in fact. The LovetoKnow Free Online Encyclopedia, at http://www.1911encyclopedia.org/ is based on the 11th edition of the Encyclopedia Britannica, which was first published in 1911. You can browse this site alphabetically or search it by keyword (the searching is a little awkward since it's administered by Google). For fun, search this encyclopedia for contemporaries of the time. Thomas Edison, for example, is noted here as having been made a chevalier of the Legion of Honor by the French government in 1878, and for his development of “quadruplex and sextuplex transmission” in telegraphy. Generally not stuff you're going to see in a twentieth-century encyclopedia. On the other hand, you can't research anybody from within the last hundred years or so in this encyclopedia. Keep it to (very) historical people and (very) historical events only.