I've been in three to four; two of which were among friends, if I remember correctly, and rest are all staff-based. First collab I've ever done was a Legend of Zelda one, between myself, Shawn (Dragon), and Derk ([supporter]Zorua[/supporter]). I was Zelda, Shawn was Link, and Derk was Ganon, I think. We've had it for only about a week. Second one was again between us three, but this time, it was on Pokémon.
First staff collab I'd ever taken part in, if I remember, was themed on GameBoy Color, where everybody'd worn forum-sets relating to old-time Pokémon games, Red, Blue, Green, and Yellow. If I remember right, I was Vulpix, because, well, I just love it! Been a fan of this Pokémon for years, and still am; it's simply adorable. Second one was on Pokémon, too, but this time, people got to have pseudo-names based on whatever set they happen to be using (again, I went with Vulpix).
Right now, I'm in a Disney collab, where all participants get to have themes depending on whatever character they choose. At beginning, I wanted to go with Ariel, but sadly, I couldn't find any good image of her, so I wound up giving her to somebody else. Instead, I picked Jack Sparrow from Pirates of the Caribbean series, and here I am!