GameUI
GameUI is the code which provides 'frontend': broadly the main menu and loading screens.
Source 2009
In Source 2009 (and earlier), GameUI is an closed engine library manipulable to a limited extent by editing .res script files. These define properties like width, height, element location, and so on. You can add new non-interactive elements, and with a bit of effort hide existing ones.
Left 4 Dead
The Alien Swarm SDK contains the complete source for the newer L4D-style GameUI, as it has been moved into the client. This version of gameui still supports the earlier Windows-like UI but in general has been redesigned for ease of use on consoles (which lack a pointing device): everything is integrated into the same navigable menu system, and you can't be in more than area at once.
The root function is CGameUI::Initialize(). Unfortunately, the server browser code is not included, and it would seem that the old DLL provides that still.