The following is a list of the tools that I have come across over the past few years which I have found exceptionally useful as a Japanese language student. I will keep this updated as I remember tools that I’ve used or discover new ones.
Undoubtedly my favourite of all the tools is a firefox extension(plug-in) called Rikaichan, and by itself this little tool is justification enough for switching to the firefox web browser over internet explorer. This tool can be enabled on any web page by right-clicking and selecting Rikaichan from the context menu, once enabled when you hover the cursor over any Japanese character or word a dialogue box will appear telling you the meaning/keyword in English, on/kun readings, and other information. Get RikaiChan here.
The next tool is a handy (or nifty) web page which allows you to search for the plain form of a verb and then builds a selection of tables displaying the various possible conjugations of the verb. The results can be displayed in romanji, kana, or kanji. Very nice and simple tool.. Japanese Conjugation Builder
ALC is next, this is a very handy site which allows you to search through web articles for idiomatic terms of phrases rather than the literal translations returned by most online translators. ALC.CO.JP
Google translate has been a surprising find whilst working on translation assignments, it is essentially a literal translation service but it also handles idiomatic translations with relative success. It also offers a quick language switch link which allows you to re-translate the results back into the original language which helps to quickly arrive at a more suitable result. Google Translate.