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.

BBS software for DOS and Windows at Jason Scott’s BBS Documentary site:
PCBoard by Clark Development Software
WildCat! by Mustang Software Inc
Worldgroup by Galacticomm

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

Archive and Disk/Disc Image Manipulation
UltraISO – CD/DVD imaging tool
WinISO – CD/DVD imaging tool
WinImage – floppy disk imaging tool
7-Zip – archiver

Drivers
S3 Trio64V+ legacy driver (for DosBox)
Sound Blaster 16 legacy driver

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