I've just released dmx485, a library and tools that speak DMX-512 over standard RS-485 adapters on Linux systems.  I've been using libdmx485 internally in another project of mine for a while, with the intention of releasing it as Free software.  The coolest part about dmx485 is that it doesn't require a special kernel driver - it works entirely through userspace terminal interfaces (termios.h and linux/serial.h).  This free version is released under LGPL 2.1 or later.  Visit the dmx485 web site to download the source code.