GNU Guix[0] supports unprivileged package management, among other features [1]. Each user may maintain one or more "profiles" containing whichever packages they'd like. On top of that, they have access to a universal virtualenv-like tool called "guix environment"[2] that can create arbitrary software environments for development or even just to do one-off experiments. It can even create containers for maximal development environment purity.

[0] https://gnu.org/s/guix

[1] http://www.gnu.org/software/guix/manual/html_node/Features.h...

[2] http://www.gnu.org/software/guix/manual/html_node/Invoking-g...