What the heck is "gununu" (ぐぬぬ)?

Fumi Yamazaki explains it quite well here:

It's not a word, it's an expression of a feeling where this character Ana in animation "Ichigo Mashimaro" expressed... she wanted to fight back but couldn't and she was going "Gununu..." or "Grrrr.......".

You can see the moment it happens here (sorry, couldn't find an english subbed version on YouTube).

So, what's Gununubooru?

Well, for some reason people latched onto the original gununu shot of Ana (>>1) and remixed it for characters from a huge range of anime. There's even some out there using characters from other mediums such as video games (>>322 and >>323 for example).

There are a few big 'pockets' of gununu pics out there (Danbooru and Sankaku Channel for example) but I'd always been frustrated that there was no definitive, huge collection of them out there.

This was something I voiced in a gununu thread on 4chan's /a/ and since several other people felt the same I decided to setup Gununubooru to start indexing and tagging all the gununu pics possible.

What can I do to help?
Upload more gununu and help tag existing gununu! :)

How can I make one myself?
There's a Photoshop template available here.