I have always been looking for some versatile tool that would help me get the fundamental framework running in a snap, when I migrate between MCUs.
mbed is an online compiler tool that addresses my need. However, mbed does not roof all the ARM microcontrollers. Nevertheless, mbed is a helpful tool when it comes to most of the reputed MCUs viz LPC1768, STM32 Series, LPC11U24.
mbed generates very helpful API library that enables one to access the on chip peripherals. This project as a package/project can be exported to be opened by Keil, IAR, CooCox CoIDE. Since the whole project cannot be built on mbed (online compiler), a standalone toolchain like Keil/IAR/CoIDE can be used to comfortably handle the project with the built in editing and compiling options.
For instance, I have been using mbed for building the base framework of STM32F103RBT6 MCU, that forms the heart of the satellite telemetry and customizable POS terminals project that I am involved in. After building the framework, I export the project to Keil and then proceed with further development on Keil v4.