Personally? Music, movies, etc (popular culture in general), politics, philosophy, languages, japanese stuff, community radio, travel, the general weirdness of humankind
Like everyone else says, you! Especially if you have an interest in popular music from other parts of the world, or you're a Japanese artist or musician.
Me: anything and everything (Tom Waits is my perennial favourite) Octopus Army: Zoobombs, Guitar Wolf, Mad Capsule Markets, DJ Krush, Shakkazombie, Buddha Brand, Nipps, Dev Large, Sunny Day Service, Advantage Lucy, Yen Town Band, Takako Minekawa, Buffalo Daughter, Namie Amuro, Pizzicato Five, Puffy, Polysics, Supercar, Acid Mothers Temple (and their 783 or so side projects!), Fushitsusha, Ruins, Boredoms, Seichii Yamamoto, Merzbow, Karafuto, Susumu Yakota, Hi-Standard, Chara, Thee Michelle Gun Elephant, DMBQ, Elle Garden, Tokyo Ska Paradise Orchestra, Skatter Brains, OOIOO, Cornelius, Nobukazu Takemura, Shonen Knife, Twigy, Miss Monday, Art-School, Kahimi Karie, Tommy February6, Yuki, Keiji Haino, You The Rock, Yellow Magic Orchestra, Toshi Kubota, Yuko Nexus6, Sawada Jyoji, Dragon Ash, Muro, DJ Honda, Grooveman Spot, UFO, High Rise, Electric Eel Shock, Bonnie Pink, Nagisa Ni Te, Fantastic Plastic Machine, Speedometer, Tha Blue Herb, KK Null, Ryuichi Sakamoto, Hoppy Kamiyama, Nicotine, Hoahio, LSD March, Limited Express (Has Gone?), Otomo Yoshihide, Ground Zero, Satanicpornocultshop, Ghost, Boris, Stars, old school garage (Spiders, Mops, Golden Cups), Flower Travellin' Band, Asian Kung-Fu Generation, Kenji Endo, Aso Ai, Piana, Mule Train, Green Milk from the Planet Orange, Utada Hikaru, Mainliner, Aiha Higurashi, Seagull Screaming Kiss Her Kiss Her, Mono, Melt-Banana, Maher Shalal Hash Baz, Takagi Masakatsu, Envy, Humbert Humbert, Soil & "Pimp" Sessions, Moose Hill and the hundreds upon hundreds of others I left out!