Disassemblers, Decompilers, Development Tools, Schematic/PCB Capture and other reverse engineering software. If you used it while reverse engineering, list it here!
Binary Ninja - reverse-engineering platform that can disassemble a binary and display the disassembly in linear or graph views.
IDA - The IDA Disassembler and Debugger is an interactive, programmable, extensible, multi-processor disassembler hosted on Windows, Linux, or Mac OS X.
Veles - Open source tool for binary data analysis (No longer actively developed).
Tools are great, and sometimes free! Without knowing how to use them, they can be a big waste of time. Better to spend your time learning the basics, then apply your knowledge.
Reverse Engineering Tutorial - A comprehensive reverse engineering tutorial covering x86, x64, 32-bit ARM & 64-bit ARM architectures.