Photo tagging - a simple application needed

One thing what I have been looking for is the easy way of tagging pictures, instead of moving items into specific folders to be able to find photos later. By tagging I can add two totally different tags, like Art and Oulu into same photo, and by doing that, find the photo among art references and among the pictures of Oulu city.

Picasa provides rather simple possibility to edit and search IPTC tags. Few days ago I was glad to hear that even my favorite viewer IrfanView also provides a possibility to view and edit IPTC data of .jpg files with just a few klicks. -It could be easier, but it's sufficient.



The surprising thing is that applications use this powerfull and potentially delightfully simple tagging system so little or create a complicated system around it..
Somebody interested in creating a nice little viewer with intuitive and effective tagging possibilities would pobably make a huge impact on the scene!