Here are 3 useful hex (firmware) files:
Eleks Maker (Benbox): LX-Nano.328p.20150626.hex
J-Tech Grbl 0.9 (optimized for laser scanning, 1kHz PWM): jtech_grbl.hex
LaserInk Grbl 0.9 by nickw89509 (optimized for laser scanning, 7.8kHz PWM): laserink_grbl.hex
The default laser pin on Grbl 0.9 was changed from D12 to D11 as it provides PWM support, if your board doesn't support this (there may be a jumper located under the Nano) and you don't want to cut traces and modify it then you will need to re-compile the hex and comment out VARIABLE_SPINDLE in config.h
I have tested these and they are stable, but if you want to ensure you're using the latest version just do a search on Github.