Plasma Next Pre-releases

AOSC OS2: http://mirror.anthonos.org/os2-repo/os2-experimental/kf5-experimental (Highly unstable and experimental packaging)
Fedora F20+: http://copr.fedoraproject.org/coprs/dvratil/kde-frameworks/
Gentoo: ebuilds available with kde overlay, packaging overview on wiki page
Kubuntu early packaging from Next PPA (This will remove your KDE Libs 4 desktop.)
Kubuntu Daily builds on Project Neon 5 (This will co-install with existing KDE Software)
openSUSE 13.1 & Factory: https://en.opensuse.org/SDB:KDE_repositories#Plasma_Next

There are also distributions that provides Live Images for trying Plasma 5, as listed here.