There are a lot of good dictionaries and reference works available online (for a simple Danish-English-Danish dictionary, click here). Some of the most useful, as far as English language is concerned, are:

The American Heritage Dictionary of the English Language
Roget's Thesaurus (the best dictionary of synonyms ever created)
Manuals of style 
 

A "manual of style" contains rules and advice for good writing. The classical and best (American) English example is a thin little book, which has been published and republished repeatedly, and can be bought for next to nothing, e.g. at Amazon.com (click to order):

Strunk, William and E. B. Whyte. 1979. The elements of style. New York: Macmillan.

The original version of this book from 1918 is available online at:

http://www.bartleby.com/141/

It's old, but it still contains some very good advice.