You can get Balloon Fight, Clu Clu Land, Donkey Kong, Donkey Kong Jr. Math', Excitebike, Golf, Pinball and Tennis from Redd's Tent, buried in the ground (villagers do this rarely), and from Nook on the last day of each month when he holds his raffle - if it's one of the prizes.
You can get Wario's Woods and Baseball from the Island - but you have to have a Gameboy Advance and a cable to link your Wii/Gamecube to the Gameboy Advance.
These 5 games were available through contests run by Nintendo: Soccer, Donkey Kong Jr., Donkey Kong 3, Clu Clu Land D and Punch Out!! You can get these by using a code generator (google it and one should pop up).
And then finally, there are 4 that you can't get without using an e-card reader or hacking: Mario Bros, Ice Climber, Super Mario Bros and The Legend of Zelda.
You can also trade with other users for Balloon Fight, Clu Clu Land, DK Jr Math, Donkey Kong, Excitebike, Golf, Pinball and Tennis