Jim Breen's WWWJDIC
Online Japanese-English & English-Japanese Dictionary with Kanji Stroke Order & Example Sentences.

Kanji Types by Grade
List of Kanji taught in the first six grades of Japanese schools, grouped by their type. A great list to make sure that you didn't miss anything ^_^

Kanji Networks
Online Kanji Etymology Dictionary

A Japanese dictionary browser plugin for Mozilla Firefox, Thunderbird and Seamonkey.

A port of Rikai-chan for Chromium/Chrome browser.

Free Japanese-English & English-Japanese dictionary program for GNOME. Includes a Kanji dictionary with handwriting (requires KanjiPad) & radical search capability. Both GJiten & KanjiPad are available from Ubuntu repositories.

A Japanese Kanji dictionary & radical elements database from the KANJIDIC & KRADFILE projects that have been converted into the OpenDocument Database (ODB) format.