You didn't mention a platform (Windows, Linux, macOS, etc), but here are some great disassemblers.
[Ghidra][1]
Ghidra is a software reverse engineering (SRE) framework created and maintained by the National Security Agency Research Directorate. Windows, Mac OS, and Linux.
Capabilities include disassembly, assembly, decompilation, graphing, and scripting, along with hundreds of other features. Ghidra supports a wide variety of process instruction sets and executable formats and can be run in both user-interactive and automated modes. Users may also develop their own Ghidra plug-in components and/or scripts using Java or Python.
IDE - Integerated (Reverse) Development Environment: Has almost anything you would normally pick IDA-Pro for (can show C source beside related ASM, Call-Graph view, multi-user collaboration). >