Furusato Matsuri (hometown festival) on now in Tokyo dome

The Furusato Matsuri collection of folk festivals, food and beer from all across Japan is on now, at Tokyo Dome.

Definitely recommended, if  you like things Japanese. You can eat local food, beer and see huge local festivals happening from all over Japan, all over the year, all in one place, over the course of about a week.
You pay admission and beer and food aren't free but it's a great experience and a great idea. There are crowds, so be prepared for that. Many breweries are there, selling full sized beers, small sizes and taster sets.
Here is a useful list of the Japanese brewers and where to find them:

A-33 Inawashiro beer.

 A-35, 36 Iwate Kura

C-53 Brewmaster

C-20 Hakusan wakuwkau beer

E-31, 32 Aqula and Tazawako

E-12 Swan Lake

E-15 Minoh

E-44 Yokohama

E-45 Fujisakura

E-46 Sankt Gallen

F-10, 11 Romantic Miura

F-8, 9Koza bakusho

D-14 Gotenba kogen

D-17 Beahern

D-36 Shiga Kogen

B-28 Abashiri

B-14 North Island

B-28 Onuma

B-2 Ishikyari banya

It is on until the 18th. Not super cheap but to be able to have everything under one roof is really cool. Enough Japanese craft beer to keep you happy, too!