With a series as big as Pokémon, you'd think that every game would be available to play or at least preserved in some fashion, but that's not the case. There's actually an official entry in the series which, at the time of writing, is totally out of reach, even for those who live in Japan (the only region it was released in).
That game is Pokémon Garden, and it might not be unplayable for much longer, thanks to the efforts of video game preservationists (thanks, Did You Know Gaming).
Only available via Yahoo Kids Japan, 2006's Pokémon Garden was an attempt to cater for those fans who had 'out-grown' the series since its inception, as well as appeal to a new generation of younger players. Obviously, with it being an online game, once it was closed, there was no way of playing it – but a group of dedicated fans are looking to fix that and make it playable in English, too.
However, this is a pretty herculean task, as Pokémon Garden was a Flash game – so the only way of tracking down the original game files is to find 15-year-old computers that still have it in their browser cache. The other issue is that one computer's cache won't supply the whole picture, so the team behind this project need community support to get it over the finishing line.
That's where you come in; if you ever played this game in the past and still have access to the computer you played it on, then you can head over here and donate data to help the project.