I track security updates for the following vendors: Apple, Cisco, FreeBSD, Microsoft, Red Hat, and Sun Microsystems. I chose these vendors based on my own needs for the networks and systems I manage. I've also found that updates from these vendors tend to catch the major updates necessary for common software applications. If you have other vendors you would like me to provide updates for, send me a message.
If you have an interest in information security from both a technical and managerial level, check out Principles of Information Security. It does an excellent job of presenting material for both audiences.