Since I aleady mentioned some mainstream Drama CDs/audio CDs over in the POLL-thread - I thought I should post about them here as well. This is original content you won’t find in the actual anime.
Some mainstream animes with audio specials, drama CDs
Gintama has several audio specials. They’re all in different formats, but you’ll feel right at home as it’s the same wacky, nerdy, fastpased humor you find in the anime and manga.
K (K Project) has one Drama CD with short sketches with the characters + a promotion talk radio show.
Naruto has at least one Drama CD released with the special edition DVD and Blu-Ray of the 10th movie: The Last: Naruto the Movie.
Shingeki no Kyojin / Attack on Titan has 9 Drama CDs made for the series.
One Piece: One Piece Drama CD: Pirate Bibi’s Adventure
Detective Conan: Drama CD Shonen Tanteidan kara no Chosenjyo!
These are just some of the top of my head, but Drama CDs are a quite common medium in Japan. A lot of light novels also get audio dramas, even if they are deemed too niche to be animated. So that’s also a tip if you’re looking for something fun or insteresting to listen to! ^>^