avahi - Add avahi/Zeroconf support
ayatana - Build in support for Ayatana notification using the libindicate or libappindicator plugin
bash-completion - Enable bash-completion support
-bcmath - Add support for libbcmath
berkdb - Add support for sys-libs/db (Berkeley DB for MySQL)
bidi - Enable bidirectional language support
big-endian - Big-endian toolchain support
crypt - Add support for encryption -- using mcrypt or gpg where applicable
cscope - Enable cscope interface -- in vim for example
css - Enable reading of encrypted DVDs
-ctype - Enable ctype functions
cups - Add support for CUPS (Common Unix Printing System)
curl - Add support for client-side URL transfer library
custom-cflags - Build with user-specified CFLAGS (unsupported)
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)
firebird - Add support for the Firebird relational database
flac - Add support for FLAC: Free Lossless Audio Codec
-flatfile - Add dbm support for flat files
fltk - Add support for the Fast Light Toolkit gui interface
fontconfig - Support for configuring and customizing font access via media-libs/fontconfig
foomaticdb - Add support for the foomatic printing driver database
imap - Add support for IMAP (Internet Mail Application Protocol)
imlib - Add support for imlib, an image loading and rendering library
infiniband - Enable Infiniband RDMA transport support
-inifile - Add dbm support for .ini files
inotify - Enable inotify filesystem monitoring support
introspection - Add support for GObject based introspection
iodbc - Add support for iODBC library
pam - Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
pch - Enable precompiled header support for faster compilation at the expense of disk space and memory (>=sys-devel/gcc-3.4 only)
pcmcia - Add support for PCMCIA slots/devices found on laptop computers
-pcntl - Add support for process creation functions
pcre - Add support for Perl Compatible Regular Expressions
pda - Add support for portable devices
pdf - Add general support for PDF (Portable Document Format), this replaces the pdflib and cpdflib flags
selinux - !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
semantic-desktop - Cross-KDE support for semantic search and information retrieval
session - Add persistent session support
-sharedmem - Add support for shared memory use
shorten - Add support for the shorten audio compressor
-simplexml - Support for SimpleXML
skey - Enable S/Key (Single use password) authentication support
slang - Add support for the slang text display library (it's like ncurses, but different)
slp - Add Service Locator Protocol support
svg - Add support for SVG (Scalable Vector Graphics)
svga - Add support for SVGAlib (graphics library)
sybase - Add support for the Sybase SQL Database Server
-sybase-ct - Add support for Sybase-CT
symlink - Force kernel ebuilds to automatically update the /usr/src/linux symlink
syslog - Enable support for syslog
systemd - Enable use of systemd-specific libraries and features like socket activation or session tracking
-sysvipc - Support for System V-compatible inter-process communication
szip - Use the szip compression library
taglib - Enable tagging support with taglib
tcl - Add support the Tcl language
tiff - Add support for the TIFF image format
timidity - Build with Timidity++ (MIDI sequencer) support
tk - Add support for Tk GUI toolkit
-tokenizer - Add support for the PHP file parser
truetype - Add support for FreeType and/or FreeType2 fonts
uclibc - Enable uclibc specific patches and build or link uclibc
udev - Enable virtual/udev integration (device discovery, power and storage device support, etc)
vorbis - Add support for the OggVorbis audio codec
wavpack - Add support for wavpack audio compression tools
wayland - Enable dev-libs/wayland backend
-wddx - Add support for Web Distributed Data eXchange
webkit - Add support for the WebKit HTML rendering/layout engine
webp - Add support for the WebP image format
wifi - Enable wireless network functions