after making a usbtiny isp programmer and using it for 6 months, i was looking at making another one for carrying around. i like the simplicy of the usbtiny isp design but would like to make it even smaller and take less parts. one thing in the original design that i want to change is to eliminate the use of clock crystal. one solution i found is that v-usb drivers supports the 16.5Mhz internal oscillator on attiny25/45/85 devices. so i start out this project to have usbtiny isp employs v-usb for usb communication. the immediate benefits is that it saves space and have less component counts (no more crystals).
description from http://www.xs4all.nl/~dicks/avr/usbtiny/
USBtiny is a software implementation of the USB low-speed protocol for the Atmel ATtiny microcontrollers. Of course, it will also work on the ATmega series. The software is written for an AVR clocked at 12 MHz. At this frequency, each bit on the USB bus takes 8 clock cycles, and with a lot of trickery, it is possible to decode and encode the USB waveforms by software. The USB driver needs approximately 1250 to 1350 bytes of flash space (excluding the optional identification strings), depending on the configuration and compiler version, and 46 bytes RAM (excluding stack space). The C interface consists of 3 to 5 functions, depending on the configuration.
description from http://www.obdev.at/products/vusb/
V-USB is a software-only implementation of a low-speed USB device for Atmel's AVR microcontrollers, making it possible to build USB hardware with almost any AVR microcontroller, not requiring any additional chip.
video on construction and usage