The best replacement for mbed is arduino.

The best alternatives to mbed are rated as following:
★★★★★  arduino
★★★  beaglebone
★★  lpcxpresso

Your alternative for mbed is not listed? Suggest!
Speaking about mbed, one probably shouldn't forget about keil.

Other search terms related to mbed and their ratings are:
★★★★★ keil
You have in mind a companion for mbed and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mbed in all years. Click on graph for more.
mbed
arduino
beaglebone