Tools
RF
Jeroen Steeman's antenna calculators
Analog
Fantastic real-world voltage divider calculator
Embedded
4046 PLL calculator
8051 timer calculator
MC34063A voltage regulator calculator
Pinout References
ESP8266 Pinout Reference
ESP32-WROOM-32 Pinout Reference
WiFi Module Pinouts
RS232 Connector Pinouts
Logic References
Interfacing 3.3V and 5V Logic
Other References
ANSI Escape Sequences
Matt's Tech Pages: Common wire-to-board, wire-to-wire connectors and crimp tools
Matt's Tech Pages: Crimp Connectors and Tools
Matt's Tech Pages: Common JST Connector Types
The Connector Book
SAS and SATA connectors
IPMI & IPMItool cheat sheet
Techniques
Multiple 5V-3.3V level conversion approaches
Libraries etc
libvterm - embedded ANSI terminal control library
National Instruments downloads (Solaris, Linux, etc)
Math
Mathomatic Quick-Ref Card
Miscellaneous
Regex Visualizer/Simulator
Regex 101
Kanji translator
Microchip/Atmel Device Support Packs
LTO tape bar-code generator