I have used QEMU to develop and debug a hobby operating system kernel. GDB and QEMU makes things so much easier than making a floppy image and booting it on real hardware (like Linus worked on Linux kernel in the early 1990's).

Making a multiboot-compatible kernel image that you can boot with QEMU (and with real iron using GRUB) requires only a few dozen lines of assembly code and a simple linker script.