ST software offerings were quite non-existant until the recent years. Due to this, the internet is full of different solutions, that different people have come up with for their needs. As of now, the System Workbench (SW4STM32) is their supported and free IDE and toolchain. The toolchain includes graphical pin planning tool, that is the best i've used so far. ST-s development boards are cheap and a lot of them have on-board stlink. Alternatively, the stand-alone st-link can be bought quite cheaply as well, it is also quite cheap and comes in a nice plastic case. For hobbyists, the aliexpress, ebay etc have very cheap STM32F103 boards and ST-link clones, it costs almost nothing to start.
NXP was one of the first to provide free development tools with Code Red's code red studio alongside with cheap development boards and on-board debuggers, that got renamed to lpcxpresso and then bought up by NXP. Lately it was renamed again to mcuxpresso and now they are adding the support for the Freescale Kinetis parts after the merger with Freescale. With the coming of mcuxpresso, the code size limit was lifted and now it is free unless you want support or some advanced features (trace). Their development boards are also quite inexpensive and some have on-board debugger. The original mbed board had nxp mcu, before ARM itself bought them.
Cypress has very good software, sadly windows-only. They have gone beyond what the others have done and a lot of the hardware related stuff can be configured with the GUI wizards. Their PSoC boards are very cheap and most come with on-board debugger.
Silabs with their former Energy Mircro microcontroller line have also a very good free IDE and nice dev boards with integrated debuggers. They are concentrating to the very-low power segment of the market, their tools have power profiling correlated to trace etc.
Atmel's studio is also a quite old piece of software (renamed from AVR studio after they added support to Cortex-M SAM controllers). Also sadly windows-only, although they run gcc under the hood. At the beginning they had re-branded segger jtag/swd probes, that were not that cheap. Now there is Atmel ICE, still quite expensive compared to others.
I'm using usually ST, NXP or Cypress parts, depending on the required functionality. I've had too many issues with Atmel (mainly with SAM9 series, but some SAM3 as well), so now we're trying to avoid their stuff if possible. Also at the last Embedded World, the Atmel was kind of un-represented at the Microchip booth and their booth was one of the most pointless booths at all.