Here is a list of all modules:
[detail level 12345]
 BoardsBoard specific definitions and implementations
 CPUCPU specific implementations
 DriversDrivers for external devices as radios, sensors, memory etc
 KernelThe RIOT micro-kernel containing the core functionality
 NetworkingNetworking libraries
 PackagesExternal libraries and applications
 SystemSystem library contains tools and utilities that make RIOT an actual operating system
 UtilitiesUtilities and helper functionality
 iolist scatter / gather IOProvides linked-list scatter / gather IO