Tools

The usual "free" set of development tools for ARM is a copy of gcc - the Gnu Compiler Collection - for your desired operating system. This is available from https://launchpad.net/gcc-arm-embedded

This is 65M+ of download (270MB+ installed), which includes C and C++ compilers and libraries for a bunch of different ARM architectures. One of the ways we simplify things is to only download the minimal set of tools needed to do something:

• arm-none-eabi-as
  An assembler.
• arm-none-eabi-ld
  A linker. The gnu assembler produces relocatable object files that need to be linked with each other, and information (start addresses, ram location) specific the the ARM chips in use. So you need a linker.
• arm-none-eabi-objcopy
  Used to produce .hex files from the linker output (.elf) files. The hex files can be uploaded to chips using common tools.
• arm-none-eabi-objdump
  Disassembler tools; useful for looking at the final (and intermediate) object files to see if the code produced looks approximately like what you were expecting.
• stm32flash
  This is a tool for uploading .hex files using the stm32f serial bootloader. It's from https://code.google.com/p/stm32flash/

Other tools

• Keil
  A commercial, proprietary C compiler/IDE company (now owned by ARM.) They have a free "lite" version that will produce up to 32k of code. This also includes a simulator, which can be very handy for debugging. Keil also includes an ARM assembler that supports the official ARM syntax.

• IAR Embedded Workbench

• Imagecraft ARM Compiler

• TI CCS

• CooCox CoIDE

• Atmel Studio

• Code Red, Atollic TrueStudio, Rowley Crossworks, Mentor CodeSourery
  These are commercial supported software development packages that use gcc compilers and other open source software, hopefully in a more polished and "easy to use" form than what you would get by putting together the various pieces yourself.