OnlineConversion.com is all by itself now in that category of Best Online Reference Sites/Tools.. http://www.m-w.com/
we'll have to add some more links, like maybe some dictionary and encyclopedias..
Online dictionary, thesaurus, and more. Better than Dictionary.com IMO, because (for just one thing), it offers free audio pronunciations. (Dictionary.com charges $20/year for its "premium" content.)http://www.noaa.gov/
Free, authoritative weather forecasts and alert information (for the U.S. only, as far as I know). A lot of other weather sites use NOAA data for their own sites. If you pay federal taxes in the United States, you already pay for it, so you may as well use it, until the idiots in congress take it away to help the third-party weather sites compete.http://www.mapquest.com/
Needs no explanation; there may be better ones, and probably not what you have in mind, but...
I'm sure someone will recommend http://www.wikipedia.org
, though I'm not sure why. The last time I made the mistake of using that waste of bandwidth, I saw a message at the top of the article that indicated the content was probably wrong, and needed review. No credibility whatsoever.