One interesting cross-domain tagging system, which I use extensively, is Fluidinfo. It allows users to attach tags, which can have typed values, to arbitrary objects identified by any unicode string (or by a UUID). There's a query language that lets you find things based on your own tags and, subject to permissions, other people's tags. It was discussed previously on /., but now has more interesting public data in it, such as most of the books from the British Library's catalogue, e.g. Animal Farm and that old /. favourite Pride & Prejudice.

Another recent development that could be significant for tagging is the announcement by Apple that OS X Mavericks will have more extensive support for tags on files both in the OS and in iCloud. Since tags look like being the only way Apple will offer to organize files in iCloud, it is possible these will catch on in a big way, and this could lead to a broader interest in tagging as a general alternative/addition to hierarchical organization.

And, of course, you're right. It seems to fail a bit more gracefully now, if you try the original ("bad") query again.

I don't think it was Fluidinfo that was misbehaving: as far as I can see it was giving a 400 error, which is OK. I think shell-fish's AJAX magic got confused, which is bad (and my fault).

Apologies again. I'll try to figure out what made it hang and fix it.

That's embarrassing: the documentation for fish was wrong.

The command should be:

show -q 'fluiddb/about = "Paris"' visited rating

If you try that, it should work in a reasonable time.

It's all my fault...I'll go and fix the documentation.