ReiserFS is a journaling filesystem for Linux licensed under the GPL-2 license and is the predecessor of Reiser4. ReiserFS was originally developed by Namesys and Hans Reiser, but is under maintenance by volunteers nowadays.
Support for ReiserFS has to be enabled in the Kernel first.
File systems ---> <*> Reiserfs support
Utilities for ReiserFS are available in sys-fs/reiserfsprogs package:
emerge --ask sys-fs/reiserfsprogs