Most countries have a 'CERT' (Computer Emergency Readiness Team) which provides both paid and free services to alert subscribers to vulnerability announcements. The general difference between the paid and free service is that paid subscribers may receive earlier notification or additional notifications via SMS etc for particularly serious vulnerabilities. These bodies often provide other value added services as well, such as summaries of significant malware, bonnet etc activity.
In addition to the CERT bodies, there are also various vulnerability database sites, such as the National Vulnerability Database from NIST https://nvd.nist.gov. Some vendors also provide similar databases such as Rapid7's database at https://www.rapid7.com/db. You may also find the open source vulnerability database useful given the systems you mentioned http://osvdb.org
Of course, if your using a single distribution, such as Debian, Red Hat etc, you will find details of security updates etc on their web sites and should be subscribed to their security update repositories etc.