MediaShare is a very good UPnP media server for the Mac. UPnP stands for Universal Plug and Play and it is a protocol that allows you to share files with other computers on your home network with almost zero configuration. In this case, MediaShare is used to stream content to devices like iPhones and iPods, the Playstation 3, and other similar devices. In theory, any device that can connect to a UPnP server will work with MediaShare. However, there are some exceptions, like the Xbox 360.
The application is very simple. The main window consists of two panes: Share and Help. On the first one, you can add folders that you want to share and turn the service on and off. You can add as many folders as you want. The Help tab shows some instructions, a description of the app and contact information.
In my testing, I connected to MediaShare from an AppleTV running XBMC. Adding MediaShare as a media source was really easy. It was detected automatically and I could browse it without problems. I then proceeded to play media. MKV and MP4 files played fine, but I couldn't open AVI files, and I couldn't play any subtitles. Maybe those are the limitations of this app. I usually share media using the WebDAV protocol and I didn't experience any limitations with that protocol.