PJRC Electronic Projects has come out with the $19 Teensy AVR development board. It uses the AT90USB162, which eliminates the need for a separate USB-to-TTL chip:
The Teensy is a complete USB-based microcontoller development system, featuring a 16 MHz AVR (AT90USB162) processor, in a very small footprint! All programming is done via the USB port. No special programmer is needed, only a standard “Mini-B” USB cable and a PC or Macintosh with a USB port.
- USB can be any type of device
- 21 I/O pins
- AVR processor, 16 MHz
- Single pushbutton programming
- 15.5K usable Flash memory
- HalkKay bootloader
- Free software tools for Mac OS X, Linux & Windows
- UART, SPI, 4 PWM, 2 timers
- Tiny size, 1.2 by 0.7 inch
- Very low cost, only $19
Teensy USB Development Board [via AVR Freaks]
Details about Arduino support, and known issues.