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 hypervison
QEMU – processor emulator
bochs – x86 PC emulator