Portage

You can search by regex using:

Uncomment the package you do not want to upgrade in world at /var/lib/portage/world

Overlay

 * Gentoo Overlays: Users' Guide
 * Layman homepage
 * browse overlays

You can search through the ebuilds available in the overlays on http://overlays.gentoo.org/ by using eix.

Defines the directory where overlays should be installed at /var/lib/layman