The Arduino ISP is an In System Programmer for any AVR board with an ICSP connection. It can be used to burn your target firmware into an AVR MCU without the bootloader, saving space on the target chip. It can also be used to put the bootloader onto a blank or corrupted AVR MCU. As well as the ICSP connection, the board also has a micro-USB port to connect to your PC. It's possible to power the target board from the Arduino ISP by setting the correct jumper. You can even re-program the on-board ATiny44.
This board is based on David Mellis' FabISP project. If you are interested in modifying the firmware then his website may be of interest.
Q. Hello. Can I use this to program a microcontroller in-circuit which is running off a 3.3V supply?
A. Hi Eric, thank you for your question. It can be used to burn your target firmware into an AVR MCU without the bootloader, saving space on the target chip. It can also be used to put the bootloader onto a blank or corrupted AVR MCU. As well as the ICSP connection, the board also has a micro-USB port to connect to your PC. It's possible to power the target board from the Arduino ISP by setting the correct jumper. You can even re=program the on-board ATiny 44. See the getting started guide here.