doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
filecaps - Use Linux file capabilities to control privilege rather than set*id (this is orthogonal to USE=caps which uses capabilities at runtime e.g. libcap)
gnome-keyring - Enable support for storing passwords via gnome-keyring
gnutls - Prefer net-libs/gnutls as SSL/TLS provider (ineffective with USE=-ssl)
idn - Enable support for Internationalized Domain Names
keyring - Enable support for freedesktop.org Secret Service API password store