A simple open-source USB AVR programmer and SPI interface. It is low cost, easy to make, works great with avrdude, is AVRStudio-compatible and tested under Windows, Linux and MacOS X. Perfect for students and beginners, or as a backup programmer.

This design is based on the USBtiny.

* Ultra low cost: programmer is $16 in parts, less than half the price of the AVRISP v2!
* Easy to build: All through-hole parts, all common and available from large distributors
* AVRdude compatible - support for usbtiny added in v5.5!
* USB drivers available for Windows using libusb, no drivers needed for Mac OS X or Linux.
* Durable off-the-shelf enclosure
* High speed! Max clock rate is 400KHz. Write speed:1Kb/s, read speed: 2Kb/s. (Atmega8 takes 8s to write, 4s to read/verify)
* 2 LEDs to indicate "USB/Power good" and "Busy"
* I/O is buffered to allow programming of 2V-6V targets (v2)
* Powered off of 5V USB bus at less than 100mA to allow it to be used with unpowered USB hubs
* Easily accessable jumper to power target project off of USB (target must be 5V tolerant, of course)
* Remove the jumper and it will self-power but buffer the I/O to match the target device. (v2)
* Easily interfaced with libusb
* Existing firmware allows for fast SPI interfacing using USB
* Bit-bang commands provide 8 bits of I/O control (including LED) for open-ended project ideas

Link: USBtinyISP

You can build this design using the schematic and firmware, or buy a kit from the Adafruit webshop.