With tons of new anime coming out every season, it's pretty much impossible to remember every show you see. Some shows have more staying power than others. No matter how old they get, anime fans aren't likely to forget Neon Genesis Evangelion or Cowboy Bebop, but the same can't be said for Time of Eve or Pandora Hearts. Of course, that's not to say that these forgotten anime shows are bad - in fact, they're actually a delight, though they lack time in the spotlight.
Just like obscure Nickelodeon shows, the anime on this list aren't the first series that come to mind when deciding which anime to recommend. Take a look at this list of older shows that slipped your mind and remember how good they were! Depending on your feelings about the anime in question, it will be a nostalgia trip, or it will remind you of why you forgot it in the first place. Vote up the anime you seriously forgot existed, but had a lot of fun watching in the past.
At one point, Trigun was about as popular as Cowboy Bebop. While you'll still occasionally hear people talk about it, for the most part it's been lost to the sands of time. Trigun is full of fun, slapstick, sci-fi adventures - until it gets seriously dark. If you remember Cowboy Bebop fondly, and you saw Trigun back in the day but are drawing a blank, it's definitely worth a rewatch.
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Outlaw Star is a show about space pirates, which is a concept that fans of anything from Cowboy Bebop to One Piece can get behind. That said, it's not a show that comes up often. While the characters are solid, the art is a bit dated and there's more filler than should be present in a 24-episode series, which might explain why people tend to forget that its existence until prompted.
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s.CRY.ed is one of those shows that most people have seen a least a couple episodes of, but didn't necessarily finish or remember. It's a decently animated show about a bizarre natural enclosure in the Kanazawa prefecture, and a mercenary named Kazuma who must survive there.
If you like characters who don't care about anyone or anything, including themselves, Kazuma might have stuck with you, but if you like your protagonists with a little more passion, maybe not so much. In addition, this cool action show might not have stood the test of time because most of the female characters are in subservient roles, a common issue with anime that fans are increasingly less willing to tolerate.
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D.N.Angel is a show so forgotten that when you look it up on MyAnimeList, the first result is actually Angel Beats. It's a 26-episode show about a mild-mannered middle schooler named Daisuke who transforms into the charismatic art thief, Dark Mousy. Said thievery isn't just done for financial gain - for a complicated series of reasons, his stealing can save the world. While the show seems to have faded from people's memories, it had a unique storyline and a vibe reminiscent of Saint Seiya and Sailor Moon.