Japanese-English Dictionary Software (also English-Japanese)
Internet-based Dictionaries
Sanseido.net offers an excellent concise Japanese-to-Japanese dictionary.
Jim Breen's WWWJDIC is a free, community built online dictionary. The definitions are not nearly as thorough as professionally compiled dictionaries but it's a good way to get the gist of a word if you need help in English.
Electronic Dictionaries (電子辞書）
The Canon WordTank
Nintendo DS Dictionary Software
漢字そのまま DS楽引辞典 I know the Nintendo DS has a reputation as a toy, but the dictionary software uses the Genius Japanese-English dictionaries-- which is what most of the other denshi jisho use as well. But because the Nintendo DS has a touch screen, you can write the kanji you want to look up, rather than the reading, so if you're totally unsure of the reading it's an easier way to check definitions. And perhaps best of all, even if you don't already own a DS, you can buy a brand new DS and the dictionary software for less than $180, which is far cheaper than most of the dedicated denshi jisho.
Quick and dirty English Guide to getting started with the dictionary.
The Mac OS X Leopard Dictionary
If you're a Mac user, and you use Mac OS 10.5 or later, you're in luck-- the new dictionary application has the Japanese-English Genius dictionaries built-in. This is the same dictionary used in the Nintendo DS software noted above, and it also powers many of the other electronic dictionaries that one can purchase.
Japanese and Japanese-English are not, however, searchable by default. To use Japanese, open Dictionary (in the Applications folder), choose "Dictionary" from the Apple menu, select "Preferences", and then check "Japanese-English" (and pure Japanese if you want).
The great advantage of this setup is that you can look up words you don't know (in the dictionary explanations) simply by clicking them with the mouse.