A widget-like panel showing you information on your WaniKani status.
Enter your API key and bookmark the resulting page.
The widget will show you the following data:
The widget will update once every ten minutes. You can change the cache time using the "Advanced options" dropdown, and you can forcefully clear the cache with the "Force reload" button in the panel. Be careful, though: refresh it too often and you will blow through the hourly API limit in no time.
The API key will be visible in the URL, so keep it to yourself! Don't embed this in a publicly available website -- I have it in an iframe in my home server, but only I can access it so it's OK.
The first post in the thread on WaniKani contains the changelog.