.LTX (LaTeX) files contain configuration data. Examples include weapon configs, graphical settings, and many other variables. .LTX files can be opened with most word processors.
.DDS (Direct Draw Surface) files contain UV and bump maps for objects, as well as decals and UI elements. .DDS files can be opened with raster editing programs such as GIMP, Paint.NET and Adobe Photoshop, although you may require specific plugins.
.OGG files contain sounds, such as ambient effects, speech, and music. .OGG files can be opened with VLC media player, and edited with programs such as Audacity. In addition to the actual sound data, most .OGG files are "commented" with metadata that controls how quickly their volume falls off in proportion to distance, and how NPCs respond to the sound.
XML (Extensible Markup Langauge) files carry in text format data related to in-game text, UI element placement, and characters and information within the game world. XML files can be opened with most word processors.
.OGF files store 3D mesh and animation data in a format unique to Stalker. Examples of meshes include weapons, NPCs, trees, and buildings. .OGF files can be opened with the .OGF viewer, and exported into Autodesk Maya and 3DS Max with the appropriate plugins.
.PPE (post-processing effector) files store data on visual effects applied over the camera. Examples include the yellow psy-field effect of SoC, and the green effect when you approach a chemical anomaly in CoP. .PPE files can be modified and created with a hex editor.
Script files store scripts that dictate various processes. Stalker uses a variant of the LUA programming language in its scripts. .SCRIPT files can be opened with most word processors.
The files all.spawn controls the placement of non-static level geometry (mutants, NPCs, items, etc) across the game world. Modifications to all.spawn never take effect until a new game is started. .SPAWN files must be decompiled and recompiled for editing with software such as ACDC.