There are two standards that new devices or apps come branded with that should ensure any client player carrying the logo can read files from any similarly certified server.
The first is Universal Plug and Play (UPnP), the long-established networking standard designed to do away with micromanaging devices on your network. You'll need a UPnP‑compatible router – but you'll struggle to find one that isn't.
Turning on UPnP in your router's control panel introduces a marginal amount of latency, but it does mean that devices are self-configuring and compatible, so the trade-off is generally worth it. Part of the UPnP standard, UPnP AV, specifically addresses the question of sharing media libraries.
Theoretically, any UPnP playback software or device should be able to read from the library of a UPnP server on the same network. Theoretically.
Depending on how libraries are structured, it can be a bit hit-or-miss as to whether UPnP devices will play together. It's not unusual for a server's library to show up within player software on another PC but appear to be empty of content. (Note, too, that not all UPnP servers and clients handle video and photos as well as music.)
Just because a network-attached storage (NAS) box claims to have a UPnP media server built-in doesn't mean its contents will automatically spring up on the screen of your UPnP TV. It's worth reading reviews that specifically mention streaming abilities before buying a networked hard drive too, as they're not always reliable.
On the upside, though, the fact that DRM-protected files are becoming extinct means that the issues surrounding streaming purchased songs are passing away too. If you've got a large library of restricted audio files and haven't tried streaming them for a while, then it's worth giving it a go again.
After UPnP, the second standard to look for is provided by the Digital Living Network Alliance (DLNA), a working group of most consumer electronics manufacturers who ostensibly want to put the bad old days of proprietary methods of media streaming behind them. The DLNA specifications are built on the foundations of UPnP, but add extra functions specifically to make sharing libraries easier.
DLNA certification is generally more reliable than vanilla UPnP, but it's not foolproof. If a shared library is set up on one device then it should appear automatically in the playback interface on another, such as the PlayStation 3.
Both UPnP and DLNA allow the server to 'push' files to clients too, playing back videos or music from your hard drive to a remote device. Unlike the PS3, the Xbox 360 isn't DLNA-compliant and streaming video to it from anything other than a Windows PC or server to an Xbox can be tricky.
If you don't want to rely on Windows Media Player, there are some good UPnP/DLNA server applications that do recognise Microsoft's games console, such as the commercial TwonkyMedia, the open-source XBMC, TVersity or Majestic.
Just don't try using iTunes unless everything in your house is Apple-branded. Its current method of media streaming – Digital Audio Access Protocol (DAAP) – is proprietary and doesn't handle video.
Not only will TwonkyMedia and so on give you access to your media files from anywhere on your LAN, they'll also transcode video files on the fly, resizing them for handheld devices and smoothing playback over a network.
If you're planning on using a dedicated PC for storing and sharing music and movies, use an OS designed for the purpose. Mythbuntu or the Live version of XMBC are simple Linux distributions, or you could invest in the all-powerful Windows Home Server. The latter will search for and archive all music and movies from any connected machine, and automatically back up other key folders as instructed.
The problem is that it's overkill. All of those tasks can be done for a lot less than the cost of a new PC by a good NAS device such as the Buffalo LinkStation or Netgear Stora. They're not as flexible as a full tower system when it comes to adding more storage, because there's limited space in the cabinet, but they're just as adept at one-touch backups and actively pulling files from any networked PC.
The latest models also give you access to your files from outside your home network, just like Home Server – although with the typical upload speeds of UK broadband still just 1Mbps or lower, don't expect to stream videos to your hotel room at any sort of quality.