It's hard to find good Wi-Fi access in Japan. Why? Please read this.
Simple answer is we don't need it and we think we should pay for that. If you pay, you can find Wi-Fi everywhere.
However, there are some companies providing free Wi-Fi these days. The list has only major ones.
Seven Eleven shops
Need registration. It seems much easier to resister it at the shops. Just connect their Wi-Fi network in a shop and follow instruction.
This one has good English instruction.
Ok, they don't have English website. Don't need registration for this one.
I will add links when I find good ones.