Here are some operating system related tools:
- Syslinux: lightweight bootloaders for floppy media (SysLinux), network booting (PXELinux), and bootable "El Torito" CD-ROMs (IsoLinux). Used by almost all distributions as media bootloader.
- Etherboot: software package for creating ROM images that can download code over an Ethernet network (x86).
- ultimatebootcd.com: bootable cd-rom with many tools, useful for wiping/cloning/restoring hard disks, cmos, bios upgrades, etc
- dban: bootable floppy that will wipe all the disks it finds, dangerous and useful all in one
- magicrescue: recover files from a broken filesystem by looking at the data
- Memtest86: Memory diagnostics tool for x86 computers.
- Linux/BSDs have many basic utilities to discover, maintain and test hardware - too many to cover here