alt Hacker NewsDDLC borrowed a lot from YOU and ME and HER: A Love Story (Kimi to Kanojo to Kanojo no Koi), which I consider generally superior to DDLC. I say this not to diminish DDLC, which is excellent, but as a plug for anyone who enjoyed DDLC and wants more mind warping content like that.
I believe Dan Salvato, the creator of DDLC, said he wasn't aware of YOU and ME and HER until late in development.