DNS

SamSpade
intoDNS
Squish
Pingability
DNS Check
DNS Report
Another DNS checker
DNS Free online tools
Network Tools
Domain Tools
Reverse DNS check

MX Toolbox
WhoIs

DNS Oversimplified

SPF Creator
SPF Record Testing Tools

BlackList Checkers

DNS BlackList checker
Multi-Realtime BlockList check
Multi RBL check
NJABL.Org
SpamHaus
Another list to check
Trend Micro MAPS blacklist lookup
SpamCop.Net
Reputation Authority
MX ToolBox - also has additional tests

Online Internet Security Testers

Shields Up
Symantec Security Check
Various Online scanners
AuditMyPc
Sam Spade online tools
SubnetOnline
CentralOps Online
Webmaster Toolkit

Open port checkers
CanYouSeeMe
YouGetSignal
ShieldsUp