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

__gnu_cxx | GNU extensions for public use
__detail | Implementation details not part of the namespace __gnu_cxx interface
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
__debug | GNU debug code, replaces standard behavior with debug behavior
__detail | Implementation details not part of the namespace std interface
__parallel | GNU parallel code, replaces standard behavior with parallel behavior
__profile | GNU profile code, replaces standard behavior with profile behavior
chrono | ISO C++ 2011 entities sub-namespace for time and date
decimal | ISO/IEC TR 24733 Decimal floating-point arithmetic
placeholders | ISO C++11 entities sub-namespace for functional
regex_constants | ISO C++-0x entities sub namespace for regex
rel_ops | The generated relational operators are sequestered here
this_thread | ISO C++ 2011 entities sub-namespace for thread. 30.3.2 Namespace this_thread
tr1 | ISO C++ TR1 entities toplevel namespace is std::tr1
__detail | Implementation details not part of the namespace std::tr1 interface
tr2 | ISO C++ TR2 entities toplevel namespace is std::tr2
__detail | Implementation details not part of the namespace std::tr2 interface