Radare2 is an open-source framework that can perform disassembly, debugging, analysis, comparing data and manipulation of binary files. It is also primarily built for Linux, but has compiled binaries on offer for Windows and macOS. It has a WebUI called Iaito.
Radare also has a debugger packaged along with it, and compared with GDB, I prefer Radare.
rax2 comes in handy when there is a need to make base conversions between hexadecimal representations, floating point values, hex-pair strings to ASCII, binary, octal, integer and so on.
rasm2 is an assembler and disassembler for several architectures, including Intel x86.
rabin2 provides key information about executable binaries. It is mostly used as a binary program info extractor.
ragg2 assists in creating shellcodes
Because it features a disassembler and a low-level debugger, it can be useful to developers of exploits. It has features which assist in exploit development, such as a ROP gadget search engine and mitigation detection. Because of its flexibility and support for many file formats, it is useful for capturing flags and other security-oriented tasks.
Supposedly it has a somewhat steep learning curve. There is a really helpful Official Radare2 Book.