CodeWeavers
http://www.codeweavers.com
Debian
http://www.debian.org
DistroWatch (for a great distribution roundup)
http://www.distrowatch.com
Linux.org
http://www.linux.org
Mandrake
http://www.mandrakelinux.com
Red Hat Software
http://www.redhat.com
Slackware Linux
http://www.slackware.org
SuSE Linux
http://www.suse.com
VMware
http://www.vmware.com
Win4Lin Web Site
http://www.netraverse.com
WINE Project
http://www.winehq.com