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
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.