X Windows

• GNOME
• Window manager site
• themes.org

Network

• SSH: Secure Shell
• Kimmo Suominen's Getting started with SSH
• Some Win* ssh clients, most of which I haven't tried: SecureCRT, PuTTY, and TTSSH.
• Be a fascist. Run junkbuster. (Note: running this on e.g. the unix servers may annoy administrators.)
• afs-faq
• Kerberos distribution

Linux / free software / commercial Linux software

• GNU and the unofficial archive.
• ibiblio, formerly metalab, formerly SunSITE
• Mirror, GA Tech
• Linuxberg
• Linuxberg's mirror in CT
• Linuxapps
• Freshmeat
• Corel
• linux.org
• linux.com
• html2ps

Drivers, operating systems, etc.

• kernel.org Linux kernel and kernel documentation
• Ethernet drivers for Linux
• Linux USB
• Linux on laptops
• Ghostscript: Postscript viewer/ translators for non-Postscript printers. UN*X, Mac, MS, and OS/2 operating systems.
• Linux and winmodems
• Windows drivers, for those unfortunate enough to...
• Free Solaris distribution, in that it costs about $10-$15 last I checked.
  if you want to use x86 for that, it's a Really Good Idea to read the hardware compatibility list. eh, maybe it supports more hardware now. (really, though , you might as well use Linux or *BSD.)
• FreeBSD, NetBSD, OpenBSD
• Debian GNU/Linux
• S.u.S.E. Linux and hardware compatibility database
• Red Hat Linux and hardware list
• List of Linux distributions

Hardware

x86 guts, mostly

• Ultra Spec Cables
• SF Cable
• Micro X-press
• Directron
• ASUS
• Insight
• Axion Technologies
• Pricewatch
• Egghead|onsale.com
• buy.com