Since their introdcution in Sonic Adventure, Chao have appeared in many other Sega products.
Playable Chao Games
Sonic Adventure (Dreamcast) - The first appearance of Chao. The player can raise Chao in the three Chao gardens of the game. One of the Main characters of the game, Chaos, is actually a mutated Chao and is treated as their guardian.
Sonic Adventure 2 (Dreamcast) - The second appearance of Chao. Chao can evolve into Hero or Dark Chao, wheras in the previous game only Neutral Chao were able to be created.
Sonic Shuffle (Dreamcast) - A party game, and you could unlock a Chao as a playable character. During battle scenes, its attack was singing to the enemy. You could not raise the Chao in this game, it was only a playable character for the board game.
Sonic Adventure 2 Battle (Gamecube) - The port to Gamecube saw the Chao system greatly modified, with an even greater amount of Chao colours avaliable to be created by the player.
Sonic Adventure DX (Gamecube/PC) - A port of the original Sonic Adventure, but keeping the newest Chao system from Sonic Adventure 2: Battle.
Sonic Advance (Gameboy Advance) - This game features a Tiny Chao Garden, where you can raise Chao on your Gameboy Advance. However the features are a lot more limited. The Tiny Chao Garden is compatiable with the gardens of both Sonic Adventure games for Gamecube.
Sonic Advance 2 (Gameboy Advance) - Like Sonic Advance, this game also features a Tiny Chao Garden. Also this game has the appearance of Cream the Rabbit, who has a Chao named Cheese. Cheese is distinguishable from other Chao by its red bow tie.
Sonic Pinball Party (Gameboy Advance) - This game also featured a Tiny Chao Garden, connectable to the Gamecube games.
Other Appearances
Sonic Adventure (Dreamcast, Gamecube/PC (DX)) - There are billboards advertising the movie "Chao in Space" around the level.
Sonic Adventure 2 (Dreamcast, Gamecube (Battle)) - There are billboards advertising the movie "Chao in Space 2" and "Chao in Space DVD" around the level. Advert for Chao Kintergarden in Kart Racing. There is also a "Find the lost Chao" mission for each level.
Chu Chu Rocket (Dreamcast) - After completing some of puzzle mode, you get unlock the option to change the Chu Chu's (Mice) into Chao.
Phantasy Star Online (Dreamcast) - In this game you can equip a "Mag" which gives you stat boosts. It is possible to gain a Chao Mag, and a RoboChao Mag (which in actual fact, is the first appearance of Omochao).
Phantasy Star Online (Dreamcast (V2), Gamecube, PC (Blue Burst), Xbox) - You can also obtain the item "Striker of Chao"
Sonic Adventure 2 Battle (Gamecube) - You can control a neutral "Chao Walker" and a dark "Dark Chao Walker" in the multiplayer section of the game. It is used as an alternative to controlling Tails' and Eggman's machines.
Phantasy Star Online (Gamecube) - You can find a lost Chao in the Gamecube version, which enables you to download a Tails Chao to your Chao Garden.
Sonic X (TV) - Episode 22 of season 1 "Little Chaos Lost // Summer Vacation Chao Observation Diary". There is a Chao Colony which is the main part of the episode.
Sonic Advance 3 (Gameboy Advance) - This game like the others, contained a Chao Garden. However it was not used for raising Chao. It is used to store hidden Chao found by the player through the main game. After all are found, you can look for keys to enter the zone's special stage.
Phantasy Star Universe (PC, PS2, XBOX360) - You can obtain the item "Chao Staff".
Sonic Rivals 2 (PSP) - Chao are a part of the main storyline. Eggman has stolen the Chao and Sonic and Tails need to save them. There is also an option in free play mode, where you need to find the ten hidden Chao in the level.
Super Smash Brothers Brawl (Wii) - You can obtain a Chao Trophy which has a bunch of normal looking chao making different expressions.
Mario & Sonic at the Olympic Games (Wii) - After playing for a while, you can see a sleeping Chao when the "take a break" popup appears. During Archery, you can see a Chao balloon.
If you notice Chao anywhere else, feel free to post at the forums here and let us know!