The most useful subnetting cheat sheet ever (IPv4 and IPv6). Expanded upon from NOW INCLUDING IPv6
MXToolbox:Utilities for diagnosing mailserver issues.
IntoDNS: An exceptional dns health check utility
Fixed Orbit: AS Lookup tools <-- lots of stale data on here, but somewhat useful still...
Filter List Generator: Prefix list generation website for various routers
SuperWhois: Advanced Lookup
Outages Dashboard:good list of ways to find "what is down" Super Cool Page with lots of utilities, including Free Access Public Telnet Servers for traceroute help, as well as LookingGlass reports and BGP Tools
Arin: Whois Lookup Page (works for IP and ASN lookups)
InterNic: Whois Lookup
KeyNote: Internet Health Report (North America Only)

PowerDNS: A very powerful DNS Server supporting multiple backend technologies and modules (yes I use it for all my nameservers)
CleanWipe: A utility for completely removing Symantec Anti-Virus when the normal uninstaller fails or generally gives you the finger.

Hardware Tools
HP Insight Foundation Download: This page has the majrity of HP Tool downloads
HP ESXi5 Download: This page launches the downloader for the ESXi image with the HP Tool preloaded
Dell R610 ESXi5 Download:This page contains the downloads for the R610 ESXi installer (works for everything dell with preloaded tools)
Dell OpenManage for ESXi 5.x and VIBs:This page contains the downloads for Dell OpenManage and the ESXi VIB
Dell OpenManage for ESXi 5.x Install Guide:Open Manage Install Guide