The source code for this program is at github.com/rrw1000/lrender. Apologies for the rudimentary controls - if you want nicer controls, let me know and I'll try to find some time make them nicer (or submit a patch, of course :-) ).
+ - Turn 90 degrees clockwise, - - Turn 90 degrees anticlockwise
F - Move forward one unit. A-E,G-R,T-Z - Macros
S - The start symbol
L-System expression after expansion