Here is a list of all documented namespaces with brief descriptions:

__gnu_cxx	GNU extensions for public use
__gnu_cxx::__detail	Implementation details not part of the namespace __gnu_cxx interface
__gnu_cxx::typelist	GNU typelist extensions for public compile-time use
__gnu_debug	GNU debug classes for public use
__gnu_internal	GNU implemenation details, not for public use or export. Used only when anonymous namespaces cannot be substituted
__gnu_parallel	GNU parallel code for public use
__gnu_pbds	GNU extensions for policy-based data structures for public use
__gnu_profile	GNU profile code for public use
__gnu_sequential	GNU sequential classes for public use
abi	The cross-vendor C++ Application Binary Interface. A namespace alias to __cxxabiv1, but user programs should use the alias 'abi'
std	ISO C++ entities toplevel namespace is std
std::__debug	GNU debug code, replaces standard behavior with debug behavior
std::__detail	Implementation details not part of the namespace std interface
std::__parallel	GNU parallel code, replaces standard behavior with parallel behavior
std::__profile	GNU profile code, replaces standard behavior with profile behavior
std::chrono	ISO C++ 0x entities sub namespace for time and date
std::decimal	ISO/IEC TR 24733 Decimal floating-point arithmetic
std::placeholders	ISO C++11 entities sub-namespace for functional.
std::regex_constants	ISO C++-0x entities sub namespace for regex
std::rel_ops	The generated relational operators are sequestered here
std::this_thread	ISO C++ 0x entities sub namespace for thread. 30.2.2 Namespace this_thread
std::tr1	ISO C++ TR1 entities toplevel namespace is std::tr1
std::tr1::__detail	Implementation details not part of the namespace std::tr1 interface
std::tr2	ISO C++ TR2 entities toplevel namespace is std::tr2
std::tr2::__detail	Implementation details not part of the namespace std::tr2 interface