Links

SSL:
https://www.whynopadlock.com
https://www.ssllabs.com/ssltest/
https://www.sslshopper.com/ssl-checker.html

DNS:
https://toolbox.googleapps.com/apps/dig/
https://www.whatsmydns.net/
https://intodns.com/
https://www.whois.net/

RBL checks:
https://mxtoolbox.com/blacklists.aspx
http://multirbl.valli.org/
http://www.anti-abuse.org/multi-rbl-check/

Vulnerability checker:
https://www.exploit-db.com/ - Everything
https://www.rapid7.com/db/search - Everything
https://wpvulndb.com/ - Wordpress

News/Articals:
http://www.linuxinsider.com/
http://www.linuxtoday.com/
http://www.linuxjournal.com/
https://www.reddit.com/r/sysadmin/
https://krebsonsecurity.com/
http://news.cpanel.com/

Tutorials:
https://www.unixmen.com/category/linux-tutorials/
https://www.digitalocean.com/community/tutorials
http://www.learnitguide.net/
http://www.itzgeek.com/category/how-tos
https://www.vultr.com/docs/