The Valve Source engine games (Half-Life 2, Team Fortess 2, Left 4 Dead, Portal) are great platform for machinima. It’s a engine that is constantly being updated and has a huge fanbase, although there are a few downsides.
Pros
Huge communities dedicated to custom content.
An advanced facial animation system.
Many maps and models to start with.
Cons
No dynamic lighting.
An ancient map editor (Hammer).
All maps and models need to be compiled.
Useful Sites and Forums
TF2Maps.net – Team Fortress 2 mapping community. Lots of useful tutorials related to map making.
Interlopers.net – A Half-Life 2 community with many technical tutorials.
Steam SDK Forums – The official Valve forums.
Facepunch Forums – The official Garry’s Mod forums.
Tools
Crafty – A Valve map viewer and file browser, with export capabilities. [Download]
VTFEdit – A tool to view and convert Valve VTF files. [Download]
GCFScape – Allows you to browse and extract files from Valve packages (GCF, VPK, BSP, PAK). [Download]
MDLDecompiler – A tool to decompile valve MDL files into SMD format. [Download]
VTA Apply – Allows you to extract VTA animations from SMD model files. [Download]
GUIStudioMDL – Used to compile QC files. [Download]
Source SDK
MDLTextureInfo [Download]
BSPSource A map decompiler for Source games. [Download]
HLLib – Command line tool to extract content from Valve package files. [Download]
Propper – Propper allows you to compile VMF brush data as an SMD file.
VMEX – VMEX is a map decompiler for Source games.
3DS Max Plugins – Wunderboy’s 3DS Max plugins which include an SMD importer/exporter and a VTA exporter.
XVI32 – A simple and free hex editor, needed for decompiling models.
Other Links
Source Recorder Tutorial
MaxOfS2D’s Source Engine Tutorials
Episode Two Scripted Scene Example