hddledPi blinks a LED on any mass storage access. Not only on SD card access, but also on USB thumbdrive and hard drive activity.
netledPi blinks a LED when there is activity on any network interface. Not only the built-in ethernet interface, but also on any other USB ethernet or WiFi interface.
actledPi blinks the Pi's ACT led on all mass storage I/O, i.e. not only the SD card.
netledPi and hddledPi make use of Gordon Henderson's wiringPi library - wiringpi.com - so you have to have that installed in order to build the programs. actledPi is a standalone, no special libraries are needed.
hddledPi uses wiringPi pin 10 by default. It is BCM_GPIO 8, physical pin 24 on the Pi's P1 header.
netledPi uses wiringPi pin 11 by default. It is BCM_GPIO 7, physical pin 25 on the Pi's P1 header.
Note: These pins are also used for the SPI interface. If you have SPI add-ons connected, you'll have to use the -p option to change to another, unused pin.