A USB programmable microcontroller is a cheap and easy way to begin experimenting with microcontroller projects. I've recently been playing with 6 USB sticks which can be picked up for under £25 ($40) and discovered three I would recommend, the Micropendous, Minimus and eZ430. Have I missed your favourite USB stick based microcontroller?
Micropendous3
The Micropendous3 (top left) contains an ATMEL AVR AT90USB647 microcontroller with 64K flash, 2K EEPROM and 4K SRAM and allows access to the MCU's 48 I/O lines. The board also supports the AT90USB1287 with 128K flash, 4K EEPROM and 8K SRAM. Support and development software are available online.
Maximus AVR USB 1.2
Despite it's name the Maximus AVR 1.2 (top right) is based on Microchip Technology's PIC18F4550 microcontroller with 32K flash, 2K SRAM and 256 bytes EEPROM. Unfortunately the board layout doesn't allow access to the MCU pins.
Minimus AVR USB 32K
The Minimus 32K (middle left) uses ATMEL's ATMEGA32U2 with 32K flash, 1K EEPROM and 1K SRAM. The board allows access to the MCU's 22 I/O lines. Development software is available online.
eZ430-F2013
Texas Instrument's eZ430-F2013 (middle right) has a detachable target board containing a MSP430F2013 microcontroller with 2K flash and 128 bytes SRAM. The board allows access to all MCU pins. The eZ430 is supplied with development tools for Windows and support is readily available online. The instruction set is easy to learn with just 27 instructions and 4 addressing modes.
Maximus AVR USB 1.0
The Maximus AVR 1.0 (bottom left) is based on the ATMEL AT90USB162 microcontroller with 16K flash, 512 bytes EEPROM and 512 bytes SRAM. As with the later version, the layout doesn't allow access to the MCU pins.
µRlink ST7Ultralite Primer v1.1
The ST7Ultralite (bottom right) is a board based on ST's 8-bit ST7FLITEUS microcontroller with 1K flash and 128 bytes EEPROM. The slightly odd looking board features a light sensor and buzzer and is supplied with a development environment for Windows. Unfortunately the IDE refused to install and there's little information online.