There has been some confusion and a lot of questions about having wireless internet in Japan. Hopefully this sticky will tame this beast into one thread.
First off, please read this wikipedia link for wireless access for your laptop:
http://en.wikipedia.org/wiki/802.11
I personally have an American Macbook (white) that uses the 802.11n spec. I've never had a problem in connecting to anyone's wireless network. Most current Macintoshes will use either the "g" or "n" spec. Check with your manufacturer if you have a Windows PC. Any router sold should have these specs listed on the box.
Second, some of you have asked about a more mobile wifi solution. Please visit these links:
Softbank
http://mb.softbank.jp/en/price_plans/packet_full.html
http://mb.softbank.jp/en/products/sharp/922sh.html
KDDI/Au
http://www.au.kddi.com/english/data/pac ... index.html
http://www.au.kddi.com/english/data/pac ... index.html
Docomo
http://www.nttdocomo.co.jp/english/prod ... index.html
Emobile
http://emobile.jp/ - Note: this site could possibly be put into a language translator website
If you have found other solutions please post them to this thread. Thanks!!