I find it surprising that there's a couple of wonderswan games for Junji Ito's Uzumaki series, although it seems like no one has ever released a fan translation of them... Also like both korean titles released for the DS, inspired by Nexon's franchises Ragnarök and Maple Story. And there's also those PS1 promo games for Ghost In The Shell and Cowboy Bebop.
Oh, and I forgot to mention that I'm completely obsessed by games made as a promo of wide-known trash food brands, like the ones for the SNES staring Chester Cheeta (shitty platform mechanics lol), the M&Ms games (my fav is the GBC one), Yo! Noid (NES), Coca-Cola kid (Game Gear) and so many others lmao...
Also love anything cartoon network-related, limited to early 2000s shows.