The current master Grbl v0.9j https://github.com/grbl/grbl
is compatible if you change the jumper under the Arduino to 0.9 or compile it with pwm disabled (comment out VARIABLE_SPINDLE in config.h).
It currently flushes the buffer on spindle commands which may cause problems with lasers (this is likely to be resolved in a future release), so you may want to use one of the modified branches that are laser specific.
Here are some of the options:
J-Tech (this is a hex you can flash to the Arduino using xloader), uses S0-255 to control the laser power by PWM.
LaserMod by idanielrobert (supports laser or cnc using $L command), uses Z0 to Z255 to control laser power.
LaserInk by nickw89509 (works with PWM or digital on/off laser), uses S0-255 for laser control when in PWM mode.
LaserMode by Andy (popular for grey scale photo engraving)
9gMod by Nefa, uses $Z# command to set laser power
There are others, I just haven't tested them yet. Please post any others you have tried and the results.
A huge thanks to Sonny Jeon (chamnit) for his support in getting Grbl to where it is now and it's continued development in the future.