How can I make my Mac a DLNA server and/or client?
There are several choices based on the fact that any UPnP server should also be DLNA compliant : 


Free ones :

XBMC
MythTV
TvMobili
Kinsky
Plex
uShare (easier to install with MacPorts)

Paid ones :

EyeConnect
PlayBack
SongBook



The bold ones are directly mentioned to be DLNA servers.