Dictionary - English
Merriam Webster Dictionary
Cambridge English Dictionary
Dictionary - Biology
Online Biology Dictionary
Dictionary - Mathematical Terms & Formulas
Dictionary - Medical
Webster’s Medical Dictionary
is an excellent free online encyclopedia. I'm often amazed at how complete it's entries are, including lesser known facts. It is also availabe in numerous other languages, like French, Spanish, Russian, etc. So it might also help you if your trying to learn another language.
is another free online encyclopedia.
LoveToKnow Free Online Encyclopedia
is based on what many consider to be the best encyclopedia ever written: the eleventh edition of the Encyclopedia Britannica, first published in 1911. At a time when many encyclopedias have capsulated and condensed important knowledge, the 11th edition is generally much more in-depth and thorough on it's topics.
Strangely enough, the 1911 Encyclopedia Britannica is the set I used at home when I was in school. It was a great resource for many projects. It just couldn't help me with questions on WWI, since the Great War hadn't happened yet.
is of course a very well known encyclopedia. The basic free service has loads of information. You may not need any more than that. However if you do, the premium service gives you access to all articles and services for a reasonable monthly or annual fee.