A few that I've come across:
KanjiConverer -
http://nihongo.j-talk.com/parser/
Converts kanji into furigana components (eg: 漢字→かんじ/ka-n-ji) to aid in reading for those with limited or no knowledge of kanji. Also supports other related conversions. Basic conversions are pretty good, but I have not tested it extensively. Potential usage: converting Japanese lyrics to romaji (eg: if you're inclined to sing-along).
Tae Kim's Japanese Guide to Japanese Grammar -
http://www.guidetojapanese.org/
A solid, concise overview of the basic components and structure of Japanese grammar. If you're starting out, this should be a good resource for learning the basics. For those taking formal classes (eg: in college), this is a nice reference. It is not extensive and there aren't many idioms or colloquialisms covered, but the explanations are quite clear.
RikaiChan -
http://www.polarcloud.com/rikaichan/
A well-known Firefox extension. Adds tooltip-style popup translations on Japanese pages by simply mousing over Japanese words. Some extra study features as well. While there is no support for specialty dictionaries, the basic covers much of what you'll encounter on the web. RikaiChan provides word translations, not sentence translations, therefore it's better suited for those with at least basic grammar and simply need assistance with vocabulary.
PeraPera-Kun -
http://perapera.wordpress.com/perapera-kun/
Alternative to RikaiChan. Supposed to be more up-to-date than RikaiChan, but some users have had problems. Give it a try if you're not happy with RikaiChan.
StarDict -
http://stardict.sourceforge.net/
Much like RikaiChan and PeraPera-Kun, but StarDict runs as a separate program and can therefore translate Japanese text from
any running Windows program, such as Outlook (Japanese emails), WinNY, various 2ch readers, and so on. StarDict
was working to provide popup translations straight off of Japanese text displayed in Firefox 2, but it hasn't been working since FF3 -- although you can still copy and paste text. However, unlike RikaiChan and PeraPera-Kun, StarDict supports custom dictionaries (proper names, scientific, medical, etc) as well as dozens of languages, making it a versatile tool. But since you can add almost
any kind of dictionary, there's no guarantee that the dictionaries you add are any good; so if you don't know which one to get, you can easily end up with poor translations.
My other suggestion: PocketPCs (Windows Mobile devices) are actually relatively popular in Japan, and as such, there is active development of Japanese-English programs. Installation can be tricky so I won't provide any links (there are a
lot of websites that claim to have the correct way to install them, but in my experience it's best to handle it case by case), but here are some descriptions of the tools:
LetsJapan and
IME-JP
Adds Japanese display and input capability for Windows Mobile/PocketPC devices. This includes being able to read Japanese emails, webpages, etc. You can also write in Japanese as you would on your Windows PC. For touchscreen devices, it also adds Japanese handwriting (full kana/kanji recognition, although it is stroke-based), meaning you can simply
handwrite in Japanese to input. See below:
JWPce and dictionaries
Much like StarDict for the PC, JWPce brings a solid dictionary experience to the PocketPC mobile device. You can add custom dictionaries (proper names, conjugations, idioms, etc). Where this shines is with the handwriting input: if you encounter unknown kanji eg on a sign or in a manga, you can simply handwrite it (using the above input) into JWPce and quickly get the reading and vocabulary translation.
I am personally an advocate of Windows Mobile for this reason. Currently I use a Windows Mobile phone (with a data plan on my carrier), and I am able to easily write and translate any kanji I encounter, search the web for Japanese information, enter Japanese addresses into Google Maps, as well as all the usual communication functions -- all from one device. Of course, not everyone needs this flexibility, or even any Japanese resources outside of the classroom or home, so don't go rushing out to buy a PocketPC. However, if you are seriously commited to JSL, and might even spend some time in Japan, I think it can be a solid investment, one which can be much more valuable than a typical denshi-jisho.