My current favorite MCU is the MSP430. In particular, I like the ones that stretch the longest lifetime out of small batteries (like a CR2032 coin cell). Unfortunately, there aren't a lot of "communication" options for such a low power design (think: how do we get logged data out of a tiny low power processor?). RS-232 is too heavy, SPI/I2C is too complex, Bluetooth and other RF is too expensive (money and power-wise).
How about a simple single wire protocol?
Dallas/Maxim has such a protocol (called One-Wire) that is very interesting, but its very proprietary (you can't legally create a slave -- they reserve the right to be the sole provider of slaves that are compliant to the protocol). Among other things, this protocol supports multiple slaves and can even be used to power them. You just need a single wire (normally pulled up) and a common ground. The single wire is used for *all* communication (bi-directional).
I'm looking at doing something similiar to this, but greatly simplified: Only one slave supported.