Nirsoft Simple Program Debugger
OllyDbg
X64dbg
SoftICE
WinDbg
Immunity Debugger
GNU Project Debugger
LLDB
ApproveAPI
API Monitor
Valgrind
strace
Process Monitor
ltrace
WinAPIOverride
JTracer
OllyDbg - OllyDbg is a 32-bit assembler level analysing debugger.
API Monitor - API Monitor is a software that monitors and displays API calls made by applications and services. Its a powerful tool for seeing how Windows and other applications work or tracking down problems that you have in your own applications
X64dbg - X64dbg is a debugging software that can debug x64 and x32 applications.
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
SoftICE - SoftICE is a debugging software for windows and DOS that analyzes all your programs and repairs them.
strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.