Ok, i start with
Jungle wa Itsumo Hare nochi Guu.
All items frome the
Guu's mistery (

,

, Manda and Chourou's Hair) Quest are from the anime
Jungle wa Itsumo Hare nochi Guu. I am an anime fan, but i never heard of it until i played Hammerfest.
If you complete the quest it says:
Effect: "This quest hasn't any practical use. It's only to suggest to you the great anime "Jungle wa itsumo Hare nochi Guu". Banyaaaaaïï ^__^"So i try it and it was pretty funny.

Thanks to MT for the suggestion.
For more information on that anime read more
here. It has one TV season and 2 OVA sequels.
I have made also a pictures from the items as they apear in the anime. Click
here.
I can go on with all the
Super Mario (Brother's). The hidden coins scattered all over HF reminds me to SMB. All the mushrooms are look like the SMB mushrooms. If you collect all four you get Marios hat as disguise and unblock a turtle shell that looks like a Coopa Troopa shell. And if you collect that, you can jump on enmies in the old SMB style. And PD16 refferes to SMB too. The name (Plumbers Hideout), the coins, the normal and the ? blocks that contains

and the green pipe.
I guess i could list more stuff but i think that's enough for one post.