There are quite a few Pokemon spin-off games that are able to communicate with and transfer specific Pokemon to main-series games.
Spin-offs that Contain Obtainable Pokemon:
In Pokemon Ruby, Emerald, and Sapphire, both Jirachi and Ho-Oh are transferrable from Pokemon Colosseum.
In Pokemon Ruby, Emerald, Sapphire, FireRed, and LeafGreen, Lugia is transferrable from Pokemon XD: Gale of Darkness.
In Pokemon Diamond, Pearl and Platinum, Mew is transferrable from Pokemon Ranch.
In Pokemon Diamond, Pearl, Platinum, HeartGold, and SoulSilver, a Manaphy egg is transferrable from Pokemon Ranger, Pokemon Ranger: Shadows of Almia, and Pokemon Ranger: Guardian Signs. Darkrai is also obtainable from Shadows of Almia and Shaymin is obtainable from Guardian Signs.
Also in Pokemon Diamond, Pearl, Platinum, HeartGold and SoulSilver, Surfing Pikachu is transferrable from Pokemon Battle Revolution.
Spin-offs and additionals that are communicable with main-series games:
Pokemon Snap and Pokemon Pinball can communicate with Blue, Red and Yellow.
Pokemon Stadium can communicate with Blue/Red.
Pokemon Stadium 2 can communicate with Gold, Silver and Crystal.
Pokemon Pinball: Ruby and Sapphire can communicate with Ruby and Sapphire.
PMD: Red/Blue Rescue Team can communicate with Ruby, Sapphire and Emerald.
Pokemon XD can communicate with Ruby, Sapphire, Emerald, FireRed and LeafGreen.
Pokemon Box communicates with Ruby, Sapphire and Emerald.
PMD: Explorers of Time/Darkness can communicate with Diamond and Pearl.
PMD: Explorers of Sky can communicate with Platinum.
Pokemon Ranch can communicate with Diamond, Pearl and Platinum.
Pokemon Battle Revolution can communicate with Diamond, Pearl and Platinum.
PMD: Magnagate and the Infinite Labriynth can communicate with Black, White, Black 2 and White 2.
Dream World can communicate with Black, White, Black 2 and White 2.
Dream Radar can communicate with Black 2 and White 2.
Poke Transporter communicates with Gold, Silver, Blue, Yellow, Red, BW and BW2.
Pokemon Bank communicates with X, Y, ORAS, Sun and Moon and U. Sun and U. Moon.
There are a ton of games (and this list is pretty long!), so please let me know if I've missed anything.
Sources: https://bulbapedia.bulbagarden.net/wiki/Event_Pokémon https://bulbapedia.bulbagarden.net/wiki/Spin-off_Pokémon_games https://en-americas-support.nintendo.com/app/answers/detail/a_id/25449/~/which-games-are-compatible-with-pokémon-bank-and-poké-transporter?