Google sites does not currently have any provision for tagging. It is a feature that has been requested by many users and I suspect that it will likely be added at some point in the future.
In the meantime, one possible workaround would be to adopt the use of specially formatted keywords which can then be located using the sites' search function. E.g.
On it's own this only addresses part of the issue as this does not provide a 'directory' of tags in use. A potential project would be to develop a script that adds some of this functionality