Are you a Linux user or do you maintain a Linux server? As time goes by, your kernel or some services may get exploited because bugs are always present in software, it's just a matter of time until they get discovered.
In the last years, the most research on linux kernel security has been done by isec.pl. Check it out and make sure you are not vulnerable to any of those exploits and advisories. Alternatively you can use grsec patch for better protection.
For a more wide variety of advisories for services such as httpd, sshd, ftpd, etc you should check packetstormsecurity.
Setup a firewall, try to keep your services as updated as possible and don't run unneeded daemons and you should be quite safe.