I like old tech. Really old tech and really old software.
WinWorld maintains a library of original install media for tons of really great and otherwise hard to find abandonware operating systems and programs for DOS and Windows 3.1, etc.
Virtualization and Emulation
VirtualBox – virtualization hypervisor
VMware Player – virtualization hypervisor
DosDox – MS/PC-DOS emulator
PearPC – PowerPC architecture emulator
Virtual PC 2007 SP1 – virtualization hypervisor
QEMU – processor emulator
bochs – x86 PC emulator
Old Linux Distros
Red Hat Linux 6.2 at Red Hat’s download archive (up to 9.0 available).
SuSE Linux 9.3 (Direct) or here at the Internet Archive
Red Hat Linux 5.2 – Red Hat official boxed release
Red Hat Linux 5.2 – MacMillanPublishing boxed release
Red Hat Linux 6.2 – Red Hat official standard edition boxed release
Caldera OpenLinux eDesktop 2.4 – Caldera official boxed release
Ubuntu 6.06 LTS