m9u is a music server, similar to mpd or xmms2. It sits in the background, waiting for commands to be delivered via a 9p file system. m9u embraces the Unix philosophy of doing one thing well. As such, it does not try to catalog or otherwise organize music, but aims for the more humble goal of simply playing it. A side effect of this approach is that m9u is trivially extensible to domains aside from music (for example, an image slideshow).
mDNS Responder with Unicast runs on a server anywhere in a network and responds to mDNS queries across the network by performing a unicast DNS lookup and returning the result. Many networks use a .local top-level domain for their intranet. This has proven to be quite problematic as operating systems such as OS X have begun handling .local domains differently (in particular, prioritizing multicast DNS over conventional unicast DNS). Ideally, you'd have no problems moving away from a .local domain name, or simply not using one in the first place. That's not always possible, though. So with this tool, a lookup for "host.local" will be resolved via your existing unicast DNS servers, even if "host" does not have its own mDNS responder. Of course, the tool isn't limited to just .local domains, and can realistically work on any network where mDNS is in use.