MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Blockchain Demo - Visual demonstration of blockchain technology
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
Crypti - A minimal cross-platform Bitcoin price widget
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Explain Me Please - Complex topics explained for your level and background