Paste Japanese text and this tool will add furigana to it. This tool additionally will show you the meaning of the word if you mouse over it and can remember which kanji you already know
Upload videos or link to YouTube. Add video, audio, or text comments at the exact place in the video where you want it. The creator can also add buttons for simpler comments, such as "awesome" or "I don't understand."
Limitations: As of 6/9/2014, the audio and video comments don't seem to work.