This very interesting site came up when I was doing a search today. (...haven't found what I'm looking for yet!)
http://www.wordchamp.com/lingua2/Splash.do
It's a site that helps you learn vocabulary. It's set up to do flash card drills. What I found exciting was that there was a lot of audio examples and you could save a custom list as an mp3 file to put on your MP3 player of choice (an iPod of course! ) Something jPod101 could think about doing maybe.
I also liked the picture flashcards where the meaning is removed from the english word to the concept itself.
Check it out.
---later on---
For people whose first language isn't English you can make lists of (for instance) French to Japanese, German to Japanese, Chinese to Japanese, Korean to... you get the idea.
It has an excellent web reader. Similar to Rikachan but it also allows you to make lists from the websites you're reading and it's many languages to many languages!
So a huge recommendation for any of your Japanese friends learning English. You also get soundfiles in the definition pop-ups.
A very exciting site I think.