Here is a list of all namespaces with brief descriptions:

[detail level 1234]

NAddonMgr
NAIRegistry
►NBattlenet
NAccountMgr
►NJSON
NLogin
NServices
►Nbgs
►Nprotocol
►Naccount
Nv1
►Nauthentication
Nv1
►Nchallenge
Nv1
►Nchannel
Nv1
Nconfig
►Nconnection
Nv1
►Nfriends
Nv1
►Ngame_utilities
Nv1
►Nnotification
Nv1
►Npresence
Nv1
►Nprofanity
Nv1
►Nreport
Nv1
►Nresources
Nv1
►Nuser_manager
Nv1
Nboost
NBuiltInConfig
NByteConverter
NCharacterDatabaseCleaner
►NConnection_Patcher
NHelper
NDisableMgr
►Nefsw
NErrors
NPlatform
NFactorySelector
►Nfmt
Ninternal
►NG3D
N_internal
Nunits
NGitRevision
►Ngoogle
►Nprotobuf
►Ncompiler
Ncpp
►Ninternal
Ninternal
Nio
►Nstrings
Ninternal
►Nupb
Ngoogle_opensource
Nhor
Ninternal
►NJSON
NRealmList
Nlfg
NMMAP
NMovement
NObjectAccessor
NOpenSSLCrypto
Nrapidjson	Main RapidJSON namespace
Nrbac
►NTrinity
NContainers
NCurrency
NHonor
NXP
►Nutf8
Ninternal
Nunchecked
NVMAP
NWeatherMgr
►NWorldPackets
NAchievement
NAuctionHouse
NAuth
NBank
NBattlefield
NBattleground
NBattlenet
NBattlePet
NBlackMarket
NCalendar
NChannel
NCharacter
NChat
NClientConfig
NCombat
NCombatLog
NDuel
NEquipmentSet
NGameObject
NGarrison
NGuild
NGuildFinder
NInspect
NInstance
NItem
NLFG
NLoot
NMail
NMisc
NMovement
NNPC
NParty
NPet
NPetition
NQuery
NQuest
NRaF
NReputation
NScenes
NSocial
NSpells
NSystem
NTalent
NTaxi
NTicket
NToken
NTotem
NToy
NTrade
NVehicle
NVoice
NVoidStorage
NWarden
NWho
NWorldState