Tier1
Tier1 contains Valve's version of the Standard Template Library, known as CUtl (high-performance game engines rarely use an operating system's own STL implementation). It also provides various other Source-specific features, most notably console commands/variables.
Tier1's source is included in the SDK at \root\tier1\. It compiles to a static library that is linked by both game DLLs.
Components
This list does not include "variations on a theme". Open Tier1's project and inspect its contents for yourself if you want a complete run-down.
General
CUtl
CUtlBinaryBlockCUtlBufferCUtlCharConversionCUtlCStringConversionCUtlDataEnvelopeCUtlDictCUtlEnvelopeCUtlHandleCUtlHandleTableCUtlHashCUtlInplaceBufferCUtlLinkedListCUtlMapCUtlMemoryCUtlMultiListCUtlNoEscConversionCUtlPriorityQueueCUtlPtrLinkedListCUtlQueueCUtlRBTreeCUtlSortVectorCUtlStackCUtlStringCUtlStringMapCUtlSymbolCUtlSymbolTableCUtlVectorCUtlFlags
