There are a few dictionary tools no student of Japanese should be without. The first amongst these is the by now quite well known Rikaichan, a dictionary extension for Firefox which, when activated, displays an inflection sensitive dictionary entry popup on mouseover of Japanese text. The original site based Rikai is now basically defunct, but deserves credit for implementing the idea.
Very little explanation is really needed of Rikaichan, as all that is required is an activation on a page with Japanese text.
Such a tool is only as reliable as the underlying dictionary, and whilst EDICT is a well developed resource, it does contain a variety of mistakes and crucial omissions, and the freely available name dictionaries are also rather erratic (though surely vastly superior to nightmarish paper name dictionaries). Using it in conjunction with an additional dictionary is probably advisable.
On the other hand, it is rather more current than published dictionaries, particularly where internet related slang is concerned, and any mistakes can be expunged (eventually) through community effort rather than waiting for a new edition of a published dictionary. The sheer speed of using it basically outweighs any disadvantages.
There are in fact a number of these tools; a look at the extensions pages for Firefox reveals a number of variations on the theme, though none perhaps as popular as Rikaichan.
The major limitation of Rikaichan is that it functions only in documents visible in Firefox. You can be cunning and email yourself .docs to a Gmail account and read them in the browser using Google’s extensive HTML conversion features, or paste text into a form, but all this is a bit of hassle. What about general Japanese documents, or even software? Try StarDict.