Free 15 Day 500MB Data Sim Card for Japan - WAmazing
Reposting this deal since it's quite old and a few things have changed since then.
Android and iPhone apps available
Register using app at least 4 hours before picking your sim card up at the airport.
1 free SIM card per account.
Additional data can be purchased, which also adds 5 day's use.
500 MB 999 YEN
1 GB 1499 YEN
1.5GB 1999 YEN
Uses Docomo network.
If a Chinese reader could dig up a promo code for extra data, that would be great.
It's not much data, but if you can use hotel wifi or Boingo with 28 degrees for most of your browsing, this may be enough for the times you're not covered.