recommended websites

Technopop:

http://park10.wakwak.com/~techno/
http://reryo.blog98.fc2.com/
http://www.artskool.biz/jem/index.html

Idol pop:

http://www.idollica.com/

General:

http://park11.wakwak.com/~rock-daigaku/
http://japanrockology.wordpress.com/
http://vacatono.flop.jp/dbx/main.php

Record labels

http://www.get-happy-records.com/Labels-Disco/WORKING-SHEETS/Japan.pdf

Free music (uploaded by artists themselves):

http://www.torumatsuura.com/
http://www.yoworks.com/yomusic/index.html
http://www.amy.hi-ho.ne.jp/dr-imai/mp3/index.html

Filesharing:

http://z7.invisionfree.com/jpopoldies/index.php?

Chinese and Korean music databases:

http://music.douban.com/ (Chinese)
http://www.maniadb.com/ (Korean)

Comprehensive recommendations:

http://www001.upp.so-net.ne.jp/tsuribe/CDs/jpope.html
http://cute.cd/azmusic/cd/txt.html

Greatest albums of all-time lists:

https://rateyourmusic.com/list/Samzpoke/takarajima__top_20_japanese_rock_albums/
https://rateyourmusic.com/list/Samzpoke/takarajima__all_picks__most_popular_japanese_albums_/
http://neojaponisme.com/2007/11/09/100-greatest-japanese-rock-albums/
http://www.kurutta.com/2009/10/150-greatest-albums-of-japanese-rock-n.html
http://www.hmv.co.jp/news/article/308150074

Greatest albums of a specific time period lists:

https://rateyourmusic.com/list/Samzpoke/100_albums_from_japanese_rock_folk_1968_1978_complete_album_guide/
https://rateyourmusic.com/list/Samzpoke/record_collectors_top_100_japanese_of_the_60s_and_70s/
https://rateyourmusic.com/list/Samzpoke/record_collectors_top_100_japanese_albums_of_the_80s/