The World God Only Knows

The World God Only Knows (aka Kami Nomi for short of romanized Japanese term) is a newer series about a master of dating sim games who tries to apply these skills to real girls with the help of a new friend.

Cameoed in
Haiyore! Nyaruko-San Episode 8

Heidi of the Alps
This series references Heidi at least twice.

There may be a third reference in a later episode.

Mario
The 2nd OVA (part 1 of 2 in the Tenri Arc) opens up with a parody of how you earn free lives using points bouncing a koopa shell against a wall.

Gundam
Episode 12 of the second series has a bath scene which parodies Sayla Mass of Mobile Suit Gundam.

Ouran
10m13s into the first Tenri Arc OVA a reference is made by the antagonist, showing the characters' parodies.

D-Frag!
13m07s into episode 5 of season 2 (episode 17 overall, "Flag.5.0 It Always Raining When We Get There", which aired in 2011) you see Keima playing on his PFP. Featured on it is a girl who resembles Takao from the 2008 manga series D-Frag, which later received a 2014 anime adaptation.

Others
In "Punch DE Date" (episode 5 of goddesses) Keima was remembering many of the galge he has played, a character that resembles Azusa Nakano (from K-On), who originally appeared in the OVA, was shown. Along with her, Yokkyun and a character that resembles Asuna, from ALO (ALfheim Online from Sword Art Online), also appeared.