I just bought a TV with DLNA support. I already have a machine dedicated to file server (with 8 disks) and I would love to use this machine as DLNA server if possible.
Are there any usable Linux implementations of DLNA?
There are several:
All implement DLNA.
There is also MediaTomb, which is a UPNP media server, though I don't think it conforms to strict DLNA, you can use it to serve media to DLNA capable devices:
I'm personally a fan of minidlna, as it's very simple. I've also personally used media tomb and coherence with success sharing photos, music, and videos to both my logitech revue, and my asus eeepad transformer.
The best DLNA MediaServer and MediaRenderer implemented out there is Rygel: