Probably, one of the most famous doujin games of Kanon might be Kanoso, which is now free to download from here.(I don’t know you can download it from countries other than Japan)
The protagonist of this game is 木目沢 ネ右一 instead of 相沢祐一 and like that, this game somehow overemphasize or break the personality of each characters. It is said that this game somehow affected to other doujin works of Kanon.
You can also download from there リトルバスターZoo (parody of Little Busters! including some Mah-jong)
If you don’t read Japanese, it also contains arranged musics of Kanon or LB. So you might enjoy them, though I don’t know its worth listening.
The writer of this game is Takumi Hiiragi ,who is also known as the author of Absolute duo(アブソリュート・デュオ), Aster, Hosizora he Kakaruhasi. I think Aster is one of well known VN which makes you cry（泣きゲー)
I found some other games
Kanon has so many. Though it seems CLANNAD or other games are popular these days, in the past Kanon defined a generation in Doujin works.
魔物ハンター舞(Mamono Hunter Mai)
Official sites of these games were closed, but you can find play videos on YouTube and some of them are sold as second-hand
Coloful Snow is Puyo-Puyo like game which you can download free from here
(All of the play videos can be found in Vysethedetermined’s channel on Youtube. And some of other key-fan games I didn’t mentioned here can be found.)
There are also some browser games like
SUPER KANON WORLD
MARIO-like game as you can know from the title.
I also found LB’s action game.
Little All Stars
Action game based on LB!