Microsoft SysInternals SDelete is a free tool by the former hacker, writer (Windows Internals) and meanwhile CTO of Microsoft Azure, Mark Russinovich. He has given trainings for Microsoft employees before MS hired him. Rest assured that he knows how NTFS works.
SDelete has several options, e.g. deleting single files, whole directories or zero the free disk space in case a file was already deleted. It is basically a single EXE file, which can be run without installation.
Downside: it’s a command line tool.
