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

Packages describing “debug” as local USE flag

Package “debug” Flag Description
dev-haskell/chaselev-deque Enable the extra internal checks.
dev-haskell/pandoc-citeproc Turn on debug tracing.
net-wireless/spectools Build the raw output client for spectools. (for developers)
app-cdr/dvdisaster Enable memory debugging.
dev-util/google-perftools Build a set of libraries with debug support (so-called debugalloc). These are available by default but are not needed unless you're actually developing using tcmalloc.
games-action/dxx-rebirth Enable various debugging code paths and sanity checks, not just output. Some of these checks date back to the retail build, and choke on some custom levels because the game was never coded to sufficiently distrust inputs
app-i18n/librime Enable debugging (including logging of input of RIME input methods)
media-tv/mythtv Instructs Qt to use the 'debug' target
app-misc/rlwrap Enable debug
games-emulation/zsnes Enable the builtin debugger
app-editors/nano Enable debug messages and assert warnings. Note that these will all be sent straight to stderr rather than some logging facility.
dev-libs/boost Build and install only the debug version of the Boost libraries. Only enable this flag if you're developing against boost.
mail-filter/dspam Enable debugging support (don't enable this unless something needs testing!)
dev-libs/openct Add debug output to the driver library for pcsc-lite.
app-arch/innoextract Enable debug logging and the --debug command-line option
app-emulation/ski enable extra binary emulation debugging via sys-libs/binutils-libs
net-misc/ptpd Enable debugging informations
net-misc/sntpd Enable analysis code path and debugging of NTP protocol.
net-misc/stargazer Enable extra debug codepaths, like asserts and extra output.
dev-lang/crystal Build each module as a separate object file. Speeds build up.
games-board/teg Turn on debugging
sci-libs/dealii Build an additional debug library with extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see <http://www.gentoo.org/proj/en/qa/backtraces.xml>
sys-apps/timer_entropyd Enable printf debugging and logging to the syslog at DEBUG level
sys-auth/pam_u2f Enable debug messages using the pam logging macros.
dev-lang/spidermonkey Enable assertions to allow for easier debugging of programs that link to spidermonkey -- note this will often crash software on regular end-user systems
app-portage/eix Build with upstream's CXXFLAGS/LDFLAGS for debugging support; not recommended for normal use.
sys-kernel/dracut Module installing additional tools like strace, file editor, ssh and more
www-servers/monkeyd Enable lots of debugging info
net-vpn/wireguard-modules Enable verbose debug reporting in dmesg of various WireGuard peer and device information.
sys-auth/pambase Enable debug information logging on syslog(3) for all the modules supporting this in the system authentication and system login stacks.
sys-process/fcron Enable debug code and output. Since version 3.0.5 this will no longer force foreground execution, and fcron will be able to run as a service properly.
sys-libs/glibc When USE=hardened, allow fortify/stack violations to dump core (SIGABRT) and not kill self (SIGKILL)
dev-tcltk/expect-lite pull in packages needed for runtime interactive debugger
gui-wm/wayfire add debugging symbols for error reporting. DISABLE ALL SYSTEM USE FLAGS BEFORE REPORTING BUGS.
dev-java/jfreesvg Enable debugging during compilation.
dev-libs/libevent Support for running in debug mode
net-misc/ntpclient Enable analysis code path and debugging of NTP protocol.
dev-perl/perl-headers Add source locations in generated .ph files to assist with debugging when errors occur
sys-block/parted Enable debugging as encouraged by upstream: [The default configuration] includes --enable-debug (by default), which contains many assertions. Obviously, these "waste" space, but in the past, they have caught potentially dangerous bugs before they would have done damage, so we think it's worth it. Also, it means we get more bug reports ;)
net-mail/vchkuser Enables debug messages to syslog
x11-wm/e16 Build libtrip debugging library

All packages providing a “debug” USE flag (1522)