Current kernels give you led0 and led1 to play with.
You can "echo none >trigger" and write 0/1 to "brightness", or you can set it to mirror any available GPIO, or …
Have fun.
Note that the green LED has a minimum on time of 0.1 seconds or so; this is so that you can see even very short bursts of activity when it triggers on the MMC controller or the network adapter. The red LED is wired directly.