The song Tokyo from the brother duo's full length debut, Enpitsu de Tsukuru Uta. If you like it, buy it. They don't get nearly enough credit. You can get their music from any of these reputable websites:
http://www.cdjapan.co.jp
http://www.play-asia.com
http://www.amazon.com
http://www.yesasia.com
Their albums are (in chronological order):
Nanatsu no Hiragana (5 song EP)
Enpitsu de Tsukuru Uta
Umikaze wa Toki wo Koete
Utaite Wo Kaete (AKA Folk Rock Guitar Duo, a collection of B-sides, rarities, & a few new songs, including the AMAZING "Yes Yes Yes")
Yukidoke No Koro Ni Todoku Tegami (new)
They also have numerous singles which also include rarities and instrumental versions. A notable single is "Kitto Santa ga" as the song cannot be found anywhere else (and it's about Santa, yo!)
They also have the DVDs "Nanatsu no Omoide Tour 2004" (live and backstage) and "Hyakka Vol. 1" (all their early videos and a "making of" for each.
You can also click the tag "平川地一丁目" to find their videos not tagged in english (and there are some good ones!)
http://www.hirakawachi1.com