Wiki Jisho

This is not a single lesson, but something which students work on and maintain over the course of the year. I have been using a site called "Wikispaces" (http://www.wikispaces.com/site/for/teachers) which offers free wikis for Educators. (I will be transitioning to a new wiki site which my school has arranged, not because I was unhappy with Wikispaces, but because it will streamline the registration and login process.)
In short, I set up a simple table on the main page of the jisho with spaces in alphabetical order. In these spaces, students can create a theme name (such as "vegetables", "pets" or "Japanese musicians"). They then create a link to a new page and begin adding vocabulary items which are related to the theme. Sometimes they add pictures and sounds as well.
In terms of class, I periodically assign students to add vocabulary to a particular theme, or to create a new theme and add a few starter words. Sometimes the assignment for older students is simply to act as editor of a given section, checking the spelling and translations of the entries made by younger students and/or organizing the vocabulary in such a way that it is easy to work with. Sometimes I will also project a section on the screen which pertains to a unit we're doing in class, and we will edit and make additions as a group.