PJRC development board

From EWiki

One of these boards powers the automated candle dipper I built a few years back, and it's an awesome toy. It's easy to program, comes with a dead-simple monitor, has tons of peripheral space, and includes expanded I/O capability with some 82C55 chips. You plug in your serial cable, fire up whatever terminal app you want, and work with the menu system to download code to RAM. I've even programmed it many times with a Palm V. It is one of my favorite boards to work with because of its simplicity and ease of use. However, the processor lacks any built-in I2C, SPI, or analog ports, so you are somewhat limited with peripheral functionality.