Kanjisaver is a screensaver that displays characters from a set of those commonly found on the JLPT.
You should install a font with Japanese character support before using Kanjisaver.
You might also be interested in Kannasaver, which is also available here on KDE-Apps.org, and upon which Kanjisaver is based.
What's New in This Release:
· New support for 'data overload,' allowing you to display both readings with or without romaji and translations.