Namecoin in itself is a peer-to-peer DNS system based off of the Bitcoin code.  You use Namecoins to register your own domain, though at the moment you need a custom DNS server in order to resolve those .bit domains.  Useful for the same reason Bitcoin's decentralization is useful, namely, that these domains can't be blocked by a governmental authority.  The project is still in its infancy right now though.

Bitparking I have no idea.