USBtinyISP kit - AVR Programmer

AVR microcontrollers programmer (a DIY kit). USB interface, 6- and 10-pin connectors for ICSP. Compatible with AVRStudio, seamlessly works with avrdude.

Do I need this programmer?

A short answer is: if you want to use not only the off-the-shelf Arduino (no matter which model), it's not necessary.

But when you are going to program microcontrollers by yourself (not equipped with Arduino bootloader, e.g. smaller ATtiny series) the USB Tiny ISP Kit would be useful. Also, when you want to clone ATmega328 and load it with Arduino, this kit will help you.

When intend to use USB Tiny ISP with Windows 7, check out this topic on Adafruit forum

USB Tiny ISP home site: http://www.ladyada.net/make/usbtinyisp/