Kanon 2006 + addThese two animes are made by the same company and that would be Key, so of course you would find the artwork exactly alike. Clannad is mysterious because of its "other world" and Kanon is about the mysteries involved in his memories. You'll find most of the heroines in Kanon having a personality like Kotomi and Fuko or the other way around. In any case, both animes are AMAZING and as a dedicated anime fan i reccomend anyone who finds one of these animes interesting to watch the other one :).

Toradora! + addBoth animes have a pretty dimwitted protagonist with a series of heroines in the plot and have a supporting character. While Clannad isn't as confusing as Toradora, the main character is still stuck between more then 2 heroines and in both animes the "unpicked" heroines are always so understanding :)

Clannad After Story + addThe is the sequel to Clannad I'm guessing it'd be just a good as the first one. I haven't finished the first one but it seems to be very interesting so far. The After story should be just as good