There is WikipediA, which is very good.
Here are a few more reference sites:
Online Encyclopedias:
| Citizendium | Digital Universe | Encyclopedia Britannica | OnlineEncyclopedia.com | FactMonster |
| Infoplease | RefDesk | Scholarpedia | Vaughn's 1 Pagers | World Book Online |

Online Library Sites:
| eBrary | The Free Library | ibiblio | Internet Public Library | LibrarySpot |
| Literature.org | The Online Books Page | Oxford Journals | Questia | WorldCat |

Online User-Contributed Reference Sites:
| Duno | eHow |Greekopedia | Instructables | Internet Encyclopedia |
| Open-Site | wikiFAQ | WikiHow | Wincyclopedia | wikiHealth |

Online Science & Math Reference Websites:
| Encyclopedia of Astronomy and Astrophysics | Encyclopedia of Earth |
| Encyclopedia of Life | HowStuffWorks | The Internet Encyclopedia of Science |
| NatureServe Explorer | Online Encyclopedia of Integer Sequences |
| Physics Encyclopedia | | PhysLink.com |

Online Fine Arts Reference Materials:
| All Music Guide | The Artchive | Artcyclopedia | DancePedia |
| Garland Encyclopedia of World Music | Improv Encyclopedia |
| Instrument Encyclopedia | |Internet Movie Database |
| Musicals101 | Oxford Music Online |

Online Language, Literature, and Word Resources:
| AskOxford | Bartleby | BookRags | BrainyQuote | Dictionary.com |
| Encyclopedia Mythica | Merriam Webster | Urban Dictionary |
|
Visual Thesaurus | WordReference |

Online Question & Answer Sites:
| AllExperts | Answerbag | AOLAnswers | Ask a Librarian |
| Askville | Microsoft Live QnA | Refseek |
| The Straight Dope | wiseGEEK | Yahoo! Answers |

Online Internet Must-Have Reference Works:
| The American Presidency Project | Bibliomania | Encyclopedia Smithsonian |
| Government Documents Center | Martindale's Reference Desk |
| NOLO Legal Dictionary | The Old Farmer's Almanac |
| The Open Directory Project | Open Library | Wolfram Library Archive |

Online Medical Reference Websites:
| Erowid | Genetics Home Reference | MedicineNet |
| Medline Plus | MedScape | Merck Medical Manuals |
| PDRHealth | PubChem | RXList | WebMD |

Online Miscellaneous Reference Sites:
| Acronym Finder | Baseball Reference | Catholic Encyclopedia |
| The Educatorís Reference Desk | FreeAdvice | Library of Congress |
| NASA Images | Snopes | Symbols.com | timeanddate |