The TI MSP430G2553 used to be available for ~$1.25 a couple years ago. Back around the time the MSP430 Launchpad was $3.99 ( complete dev board and micro USB cable included ). Since then though the board is now $10.99, and I think the MCU's are now selling for $3.00 ea in lots under 1ku.
I'd still consider it a good micro for the fact that if I recall correctly it only needs a single cap and resistor to get running. Has an onchip watchdog, ADC, SPI x 2, I2C( if you forgo one SPI ), and a couple timers / PWMs. Oh, and right a UART too. All this in a package that can literally run for years off a button cell battery. It's nothing special with only 16 pin outs( in DIP package ), and up to 16Mhz. But if you need a small power efficient micro for a limited task it can come in real handy.
Lately though I've gone the other way. I'm using a beaglebone black + CAN cape to read from our Schneider inverters . . .
EDIT: Ah and I forgot to mention the MSP can be programmed using TI's code composer studio, but one of the major attactions for me on the software side. Is that you can use gcc, and other opensource software to write code, and upload to it. There is even an MSP430 port of gdb . . .