There are dozens of promotions in Japan, ranging from national to backyarders. I won't be listing all of them, since they are harder to follow, so here are the top six promotions in Japan in no particular order:

New Japan Pro Wrestling

Shinsuke Nakamura	Jushin "Thunder" Liger	Koji Kanemoto	Yuji Nagata	Hiroshi Tanahashi
Tiger Mask	Hiroyoshi Tenzan	Ryusuke Taguchi	Manabu Nakanishi	Yujiro

Founded in 1972, New Japan Pro Wrestling is one of the largest promotions in Japan. They have a strong Heavyweight and Jr. Heavyweight division and run events all across the country. Their events are available on a number of television stations in Japan and they run occasional PPVs. They have two big events during the year: an annual Tokyo Dome event on January 4th, and the G1 Climax round-robin tournament in August. In 2005 Yukes brought out New Japan, and with Inoki losing control the style changed to less of a focus on hard strikes and flash submissions. The structure today is mostly faction-based, with clear faces and heels. They have four sets of belts: IWGP Heavyweight Championship, IWGP Heavyweight Tag Team Championship, IWGP Jr. Heavyweight Championship, and the IWGP Jr. Heavyweight Tag Team Championship.

All Japan Pro Wrestling

Kohei Suwama	Akira Raijin	Joe Doering	Keiji Mutoh	Ryuji Hijikata
T28	Satoshi Kojima	Shuji Kondo	Minoru Suzuki	"brother" YASSHI

Founded in 1972, All Japan Pro Wrestling has changed considerably in the last ten years. With Giant Baba as the booker the matches were more structured and saw the rise of Misawa, Kobashi, Kawada, and Taue. After Giant Baba's death, however, the majority of the big stars soon left and formed Pro Wrestling NOAH. Keiji Mutoh was hired as President of All Japan in 2002, and since then the promotion has had a more "sports entertainment" feel to it. All Japan uses a fair number of American wrestlers (foreign wrestlers in Japan are called gaijin) and Freelancers, but do have a set number of contracted wrestlers. They also have factions, although not as pronounced as New Japan, with heels and faces. Every spring they hold the Champion Carnival, a round robin tournament. They currently have two active belts: The Triple Crown Championship and the All Japan World Jr. Heavyweight Championship.

Pro Wrestling NOAH

Kenta Kobashi	Go Shiosaki	KENTA	Akira Taue	Jun Akiyama
Naomichi Marufuji	Yoshinobu Kanemaru	Takeshi Morishima	Takeshi Rikio	Mitsuharu Misawa

Founded in 2000, Pro Wrestling NOAH was formed when Mitsuharu Misawa split away from All Japan and took the bulk of the roster with him (the only Japanese wrestlers that didn't make the jump were Kawada and Fuchi). They also took the style with them, and even though its not exactly the same as All Japan in the mid-90s, since the stars of NOAH are the same as the former stars of All Japan (Taue, Misawa, Akiyama, and Kobashi) the style is similar. They also have an electric Jr. Heavyweight division which was originally boosted by KENTA and Marufuji in 2003. NOAH books foreigners on every tour and frequently works with Ring of Honor. Heels and faces are more blurred, with very few wrestlers considered serious heels at any given time (such as SUWA before he retired). They currently have five championships: GHC Heavyweight Championship, GHC Heavyweight Tag Team Championship, GHC Jr. Heavyweight Championship, GHC Jr. Heavyweight Tag Team Championship, and the GHC Hardcore Openweight Championship.

Big Japan Pro Wrestling

Abdullah Kobayashi	Jaki Numazawa	Jun Kasai	Ryuji Ito	Takashi Sasaki
Yuko Miyamoto	The Winger	Daisuke Sekimoto	GENTARO	BADBOY Hido

Founded in 1995, Big Japan is currently the only mainstream promotion that has a focus on death matches. Most famous for their light tube matches, which they have several of a month, the promotion also has a fair number of wrestlers that excel without doing the death match style. They have a bright crop of young talent, including Miyamoto and Sekimoto, along with innovated and insane death match wrestlers such as Kasai and Numazawa. Big Japan has a healthy heavyweight and tag team division, but currently lacks a strong Jr. Heavyweight division. They currently have two championships: the BJW Death Match Heavyweight Championship and the Big Japan Tag Team Championship.

Dragon Gate

CIMA	K-ness	Anthony W. Mori	Masaaki Mochizuki	Dragon Kid
Susumu Yokosuka	YAMATO	BxB Hulk	Masato Yoshino	Naruki Doi

Dragon Gate has a long and storied history. After starting as Toryumon with Ultimo Dragon as their trainer, in 2004 they split away from Ultimo Dragon and formed "Dragon Gate." The promotion is very faction based, with almost all of the feuds dealing with one faction battling another faction. Heels and faces are clearly distinguished. The matches are generally fast paced, especially the six man tags, and the tag rules are very loose. Of all the promotions in Japan, Dragon Gate perhaps has the most diverse style. CIMA has been the leader of Dragon Gate for several years but has a solid group of young stars ready to take over. They currently have four championships: Open the Dream Gate Championship, Open the Triangle Gate Championship, Open the Twin Gate Unified Tag Championship, and the Open the Brave Gate Championship.

Hustle

Hard Gay	Real Gay	Genichiro Tenryu	Ryouji Sai	Wataru Sakata
Bob Sapp	Commander An Jo	The Esperanza	Toshiaki Kawada	Bono-chan

Founded in 2004 by Nobuhiko Takada, Hustle is the most sports-entertainment oriented promotion in Japan. Originally owned by the same company that owned Pride, the promotion was known for putting on events in large auditoriums with a show that was as much "entertainment" as it was wrestling. Like most promotions in Japan, Hustle is also faction-based, with the HUSTLE-GUN battling the TAKADA MONSTER-GUN. Hustle doesn't take itself too seriously as the matches occasionally end with special effects or comedy elements. They did help create one of the biggest wrestling sensations in years however with the rise of "HG" Hard Gay. Even though Dream Stage closed down, Hustle continues to strive and books major Freelancers such as Sapp, Kawada, and Tenryu on a regular basis.

Other frequently running promotions include Zero-One MAX (led by Masato Tanaka, Shinji Otani, and Takao Omori), Michinoku Pro (led by Great Sasuke and Jinsei Shinzaki), and Osaka Pro (led by Super Delfin, Tigers Mask, and Billy Ken Kid).