The ARM Cortex-M3 is the baseline of ARM 32-bits microcontrollers. Many products spawn from its design: the Cortex-M0 (a stripped-down version for small systems), the Cortex-M1 (specifically designed to be implemented in FPGA) and the Cortex-M4 (with Digital Signal Processing extensions).
One chip family that contains the Cortex-M3 is the STM32. STMicroelectronics recently introduced the Discovery Kit: a very low cost evaluation board that can be easily connected to custom electronics. The processor is a 24MHz Cortex-M3 and the peripherals include many standard low-speed ports such as UART, I2C and SPI, and plenty of GPIOs. The development tools that are ready to be used for this platform are Atollic TrueStudio (Lite is free), IAR Embedded Workbench and Keil development kit.
Another low-cost board that contains the STM32 is the Maple. It sports a 72MHz processor and many peripherals. The board design is open source, as well as the development environment, that is derived from the Arduino IDE. Make Magazine wrote some thoughts on it, comparing it with Arduino, while LeafLabs already produced an application that implements audio effects.