The Freescale Freedom development platform is a low-cost evaluation and development platform featuring Freescale's newest ARM Cortex-M0+ based Kinetis KL25Z MCUs. The board comes with Arduino compatible headers which mean any 3.3 V or 5 V Arduino shields that fit the Uno footprint should be compatible with the Freedom Platform. The Cortex-M0+ MCU runs at 48 MHz and should be a good first experiment with 32-bit architecture if you have only used 8-bit MCUs before. You can use Codewarrior or Keil IDEs to program the board, or open-source equivalents.
http://www.element14.com/community/community/knode/dev_platforms_kits/element14_dev_kits/kinetis_kl2_freedom_board
http://tr.farnell.com/freescale-semiconductor/frdm-kl25z/freescale-freedom-hardware/dp/2191861