hddledPi blinks a LED connected to a GPIO pin on any mass storage access. Not only on SD card access, but also on USB thumbdrive and hard drive activity.
netledPi blinks a LED connected to a GPIO pin 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.
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. Current versions of Raspbian come with wiringPi already installed.
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.