alsa - Add support for media-libs/alsa-lib (Advanced Linux Sound Architecture)
dbus - Enable dbus support for anything that needs it (gpsd, gnomemeeting, etc)
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
g15 - Enable support for the Logitech G15 LCD (and compatible devices).
jack - Add support for the JACK Audio Connection Kit
multilib - On 64bit systems, if you want to be able to compile 32bit and 64bit binaries
nls - Add Native Language Support (using gettext - GNU locale utilities)