/var/cache/binpkgs
From Gentoo Wiki
/var/cache/binpkgs is the directory Portage uses to store prebuilt (binary) packages. This directory is defined in /usr/share/portage/config/make.globals and can be altered by changing the value of the PKGDIR variable in make.conf.
See also
- Eclean — a tool for cleaning repository source files and binary packages.