These tools help turn your mouse into a dictionary. It is very addictive and can really help speed up your reading - but the lack of pain may mean that you don't remember kanji or words as well as when you look them up one by one.
I think that these are my top picks for free online dictionaries. You may have other suggestions, please let me know and I'll add them.
Sometimes you really need to use a special dictionary to find particular words. Here are some examples: onomatepeia, slang, etc.