This will be a constantly-updated list of sites that I visit often.
OUI Lookup (WireShark) – Find what manufacturer a MAC belongs to
Packet Life – Great reading, cheat sheets, and more!
Cisco Power Calculator – See how much juice you PoE devices will consume
Cisco IOS Vulnerability Checker
Subnet Calculator – For the lazy
Warriors of the Net – Blast from the past
A reminder of the past… – EEEEEE ERRRRR KSSHHHHHHHHH BLEEP BLEEP BLEEP…
Stack Overflow – Support community for programmers
JSFiddle – Test JS and libraries
Microsoft Resources/Downloads – Microsoft-y apps/resources
Microsoft ScriptCenter/Repo – Microsoft-y user-submitted scripts
PoshCode – PowerShell Code Repository
Spiceworks Script Center – Collection of user-submitted scripts/tools
retroUSB – Custom console and homebrew games