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.
These websites provide reference documentation of the x86 instruction set: