Jesus Echavarriahave built pretty handy development board based on Atmel AT89C51RE2 microcontroller. This is a small board that packs few features that enable fast prototyping. First of all it is powered at 3.3V using low dropout regulator. So it can be powered from 5V USB port. Microcontroller is 8051 architecture with 128KB of Flash and 8KB of RAM.
Development board carries TUSB3410 chip from Texas Instruments, which converts MCU serial interface to USB. It can be used on PC as virtual COM. There are also couple switches, buttons, LEDs and buzzer. They can speed up prototyping significantly. The list continues with External I2C bus which his isolated using ADUM1250. It also works as voltage level convertor from 3 to 5V. The nthere is a standard 1-wire temperature sensor DS18B20. To internal I2C interface there are RTC, EEPROM and ADC connected. As you can see it is quite powerful collection to do some serious projects.