The most useful subnetting cheat sheet ever (IPv4). (Swiped and expanded upon from krow.net)
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"
TraceRoute.org: 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)