OSdata - Operating System Technical Comparison, a very interesting site.
RFC editor - RFC database.
GPG - open source replacement for PGP.
Unix Guru Universe - one of the best sites on Unix system administration.
TLDP - the linux documentation project. Many very useful linux docs.
Slackware - my favorite linux distro. Stable, reliable and flexible (and bloat free).
Shelldorado - the best resource on Unix shell scripting on the net.
SecurityFocus - a large site on everything computer security related.
SecLists - archives of some of the best security mailing lists.
Cryptogram - Bruce Schneier's popular cryptography/security newsletter.
Packetstorm Security - a very well organized site of up-to-date and historical security tools, exploits, and advisories.
Pouet.net - demo scene online community.
A.D.A. - Amiga Demoscene Archive.
Gfxzone - the ultimate demo scene graphics site.
Nectarine - demo scene music radio station.
Back2Roots - Amiga Culture Directory Project.
Lemon - classic Amiga games community and nostalgia.