The Godbolt compiler explorer allows the user to compile a function and see the corresponding assembly code. It can highlight matching parts in the language and assembly, making it easy to understand how individual expressions are compiled. It supports common languages like C, C++, Go, Rust. It can also work with assembler and LLVM IR.
Another nice feature is, that it can show statistics about assembler code, like needed cycles, instructions, and which resources the instructions need. This uses the LLVM Machine Code Analyzer.
These websites provided an overview over the Linux systemcall interface by listing the syscall numbers, their meanings, and their arguments.
Website allowing assembly and disassembly of x86 and x64 code.
The online disassembler can disassemble a wide range of binary formats and platforms. It can disassemble free-standing bytes too. It provides a disassembly view, a graph view, a symbols and a function viewer.
Rappel is a pretty janky assembly REPL. It works by creating a shell ELF, starting it under ptrace, then continiously rewriting/running the
.textsection, while showing the register states. It's maybe half done right now, and supports Linux x86, amd64, armv7 (no thumb), and armv8 at the moment.
These websites provide reference documentation of the x86 instruction set: