This programmer is based on Thomas Fischl's USBasp design and connects to your computer's USB port.  Not only is it quite compact, but the design is really elegent.
The USB interface is achieved by using an atmega processor and the rest is done in firmware. For those interested, the firmware source code can be downloaded from the USBasp website.
Some of the features include:
Support for AVRDude from version 5.2 onwards
Allows you to to read or write the microcontroller EEPROM, firmware, fuse bits and lock bits
Support for Linux, OS X and Windows
5 KB/sec maximum write speed
Software controlled SCK option to support targets with low clock speed (< 1.5MHz)
10 pin ISP interface
These programmers are a perfect companion to the Protostack 28 pin AVR Development Board or the Protostack ATMEGA8 Development Kit.
Supported microcontrollers include:
Mega Series
ATmega8 ATmega48 ATmega88 ATmega168 ATmega328
ATmega103 ATmega128 ATmega1280 ATmega1281 ATmega16
ATmega161 ATmega162 ATmega163 ATmega164 ATmega169
ATmega2560 ATmega2561 ATmega32 ATmega324 ATmega329
ATmega3290 ATmega64 ATmega640 ATmega644 ATmega649
ATmega6490 ATmega8515 ATmega8535
Tiny Series
ATtiny12 ATtiny13 ATtiny15 ATtiny25 ATtiny26
ATtiny45 ATtiny85 ATtiny2313
Classic Series
AT90S1200 AT90S2313 AT90S2333 AT90S2343 AT90S4414
AT90S4433 AT90S4434 AT90S8515
AT90S8535
CAN Series
AT90CAN128 PWM Series AT90PWM2 AT90PWM3

-> Download the Instruction Manual

Sample Project: Burning bootloader on ATMEGA328P using USBASP AVR Programmer