 If you liked
Tsubasa: Tokyo Revelations
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 ...then you might like
Kyou kara Maou! R
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It's completely different in every way but one - they do not fit in to the series story-line. It's just random extra that you don't know what to do with. In "Tokyo Revelations" case it's a part of the true story-line from the manga which doesn't fit into the fluffy story-line of the anime. They're both worth a watch ("Tokyo Revelations" more so) but when you go back to the real series you have to forget everything that has happen. They're both good places for spoilers as well so I recommend watch after you've seen and/or read everything~ else 0:-)