I use FastStone. It has all the features I would ever want in an image viewer (and more), except the ability to search for images based upon content. Does anyone know of an image viewer that lets you do this?

What I'd like to do is to put all my images into basic folders (like the name of a character, or the name of a doujin, for instance), add tags like we have here in Gelbooru, and then be able to call up a display of all the images which meet those requirements.

I've been able to find nothing at all with this capability, and while I could probably whip up a rough program in VB.net to do something like this, I just don't have the time to write and debug something that extensive.

You could do that in ACDSee - it has quite decent keywords/category support. Or if you need something more powerful, try some digital asset management software, like Microsoft Expression Media, or Extensis Portfolio.