Boogiepop wa Warawanai, Ergo Proxy Recommendations
I think the feeling in both is the same; both of the animes are surrealistic and dark. The music in both is very good and somewhat similar.
Ergo Proxy and Boogiepop Phantom most noticably have their atmosphere in common. While it isn't the same, Ergo is told in mute, airbrushed and dim, cold colours, while Boogiepop is dominently sepia-toned and 'glowy', yet dark and bleak. Also, the two have intelligent plots and require time, patience and attention to detail if a viewer is going to get the most out of either of
Spooky confusing atmosphere is in both. Neither resolves the plot in a super compelling way. Boogiepop feels like there is a lot of unresolved question, but I think it works in that particular anime.