The online disassembler can disassemble a wide range of binary formats and platforms. It can disassemble freestanding bytes too. It provides a disassembly view, a graph view, a symbol and a function viewer.

Rappel is a pretty janky assembly REPL. It works by creating a shell ELF, starting it under ptrace, then continuously rewriting/running the .text section, 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.