PC to controller comunication protocols

Hi all. For a while I have been working on a microcontroller program to take
in g-code sent over a serial line (COM) and translate it to machine
movements. It's going ok so far, it can take basic g-code such as would be
created by eagle for PCB milling (user contrib script, cant remember who but
thanks a lot) but it's causing a big hold-up to implement the more
complicated functions such as curves (bloody floating points :) ).
One plan is to put an interpreter on the PC to take in any g-code that
confirms to the spec and turn it into simplified code that the
microcontroller can handle as-is.
This will still used lines of g-code sent one by one directly over the serial
line (ascii). The idea is any PC can be plugged into it and work with no
special software, just hyperterm, gtkterm or... whatever it is macs use.
Can anyone suggest an alternative to ascii-over-serial? It needs to be a
common and free to use protocol that doesn't lock the system into one
specific OS or software component. I have had a look around but have only
seen either systems for direct control over the parallel port or
vendor-specific systems that keep their protocols close to their chest.
If something like that exist then I'm at the point where it would make sense
to use it, otherwise I'll stick to ascii-over-serial.
Thanks.