Sites I’ve come across that are really good resources:

VMware – PowerCLi VMware scripts

Networking – Network & DNS checker – IP, DNS, & Blacklist checker

Cisco – Any documentation and downloads Cisco related can be found here

Microsoft – MS news and tutorials – Anything AD related – Application Dependency mapper

E-Mail – SMTP testing tools – MS Exchange blog

Security – Free 90-Day SSL Cert – Free 90-Day SSL Cert

Blogs – VMware vSAN Blog – VMware Blog – General IT blog – General IT blog