Cosumi is a multi-lingual web-based Go-playing software developed in Japan. It can play at board sizes from 5x5 to 19x19 and with handicaps from 0 to 9 stones. It uses the Japanese rules. According to its FAQ, its playing strength is around KGS 8k.
PS (Sept 2022): Now it is much stronger, Level 8 is at high-dan level. One level difference corresponds to 80%/20% win rates. Flash version was discontinued.
HTML5 version: http://www.cosumi.net/en