NirSoft is old and popular site maintained by its’ owner and developer Nir Sofer. There are dozens of apps available there - they are commonly written is C++, small and portable GUI utilities (there are some CLI ones/versions as well). Most popular probably belong to password-recovery and Internet categories while system ones tend to have some power user (or just strange :) ) functionality.
Unlike Sysinternals site doesn’t offer complete utilities package for download but I doubt a lot of people may need all of them. It’s more like great place to find utility with some very rare and specific function that no other person ever bothered to code.