Jigoku Shoujo Mitsuganae

Both series conveys the theme of death, its aftermath, the consequences that may have caused it, and ones own faults. Additionally, both anime(s) have an eerie-like soundtrack that reflects the view of the mysterious atmosphere of the show. For those who enjoy psychological series and thriller themes, these two series are a must-see. 

Both titles show the unfairness of life and the ugly side of our fellow humans. Towards the ending, Mitsuganae displays even more concrete parallels to Hotaru no Haka. I don't want to become too specific (that would be spoiiling), but they both run towards a similar, depressing finale. Both made me think that killing by neglect may be the cruelest way of killing someone... If you suffer from too much happiness in your life, both titles may ease your pain. (I wonder if these could be counted among the opposite of Iyashi Kei) 

Besides the obvious similarity of the use of websites for making deals for supernatural power, the protagonist of Mahou Shoujo Site has some similarity to the second protagonist of this season of Hell Girl. 

Both Boogiepop Phantom and Jikogu Shoujo feature 'victim of the days' who cannot escape their fates. However, as both series progress, a deeper story unfurls. Both anime are also very dark and moody. 

The animation and artistic style of this reminds me of Madoka Magica. They also both have a dark plot and both have a girl as the main character. They both play on the idea of the innocent being vulnerable to corruption and of evil being disguised as innocent.  

The atmosphere in both these animes feels somewhat the same, the case by case problem solving also feels very similar together with the dark setting of the whole anime.  

If You Like Hell girl or any of the other Sequle's you wil like Yamato Nadeshiko Shichi Henge - also known by The Wallflower 

The 3rd season of Jigoku Shoujo now gives more emphasis on how injustice annihilates justice. Both series deal with morality that ranges from white to black. In both series, there is no absolute good or evil, mostly everything is debatable. Both series focus on human morality (mostly dark shades of gray). ... Both series show that humans are bastards. There's a lot to be learned from watching these shows. 

I recommend this, not because of the storyline itself, but because of the main characters. There's mystique around them and they both have a mission. It's hard to explain without spoiling, but Jigoku Shoujo is never a bad choice, so just watch it anyways ^__^