MSP430

The MSP430 microcontrollers are Ultra-Low-Power mixed signal, 16-Bit RISC Core devics.
They an run up to 8Mhz (up to 16 Mhz for the new MSP430F2xxx series) and their consumption is only 250 uA per MIPS, which makes them perfect for portable and handheld devices.

MINUS:
Von Neuman architecture, instructions take from 1 to 4 bytes, and are performed in 1 to 6 clocks.

PLUS:
Very low power: 250 uA per MIPS.
Mature product with very clean architecture - code porting between different members of MSP430 family needs alsmost no re-touch.
Avaliable in any quantity through TI distributors, with lots of application notes and code examples written.
Very good range of peripherials: Precise ADC and DACs, Comparators, OAMPs on chip, LCD drivers, SPI, UART, DMA.
The new members MSP430X have extended instruction set and allow more than 64KB memory addressing.

MSP430 allow development with completely free compiler and IDE setup with MSPGCC + Eclispse:

Setup MSPGCC+Eclipse

Step-by-Step programming guide for MSP430 under Linux

Building up to date toolchain for MSP430

Eclipse and MSPGCC under Windows

Debugging MSP430 with Eclipse

MSP430 Launchpad and Linux

MSP430 development in Linux

and again MSPGCC

MSP430 and GCC FAQ

MSPGCC WIKI

Linuxjournal MSP430 debugging article