Keil originally developed one of the first C compilers for the 8-bit 8051 architecture back in the 1980s. The company moved on to support other cores and was eventually acquired by ARM. They currently offer one of the most efficient compilers for ARM Cortex-M devices (Clang/LLVM). A free version of the uVision IDE is available but is limited to 32 KB code space. Various tiers of the IDE are available in several licensing options (such as perpetual, subscription-based, and so on). Modules of code are added through software packs, which simplifies rapidly setting up projects. A very fully featured middleware stack is available as a top-tier offering, which comes abstractions for different RTOSes as well as a uniform API on top of all of the supported MCUs. 

Here are the quick stats for uVision:

FreeRTOS task-aware debugging is not available—Keil uVision has elaborate support for their own freely available CMSIS RTX RTOS instead. The code editor in uVision MDK is also overdue a facelift. 

Similar to Keil uVision, IAR Embedded Workbench is another long-standing IDE for embedded work.