LFS - a semantic file system

Very interesting concept: read more here.

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One of the main idea in LFS is that files have many properties.
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For instance, when it indexes /usr/include/stdio.h it will internally assigns the properties name:stdio/, ext:h/, size:10Ko/, date:30-Jun-2005/, dir:include/, and dir:usr/ to this file. It will also assign the file the properties include/ and usr/ which will later allow the user to also navigate using familiar names. So, traditional directories are just a special case of properties, and thus LFS generalizes a classical hierarchical filesystem. Indeed, the LFS properties are a little bit like tags in youtube. Each file in LFS has many tags.
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With LFS you can query your files using operators of traditional boolean logic.
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So, with these boolean operators, it's like you can google your filesystem.
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And a tutorial here.