The OES_vertex_array_object extension is part of the WebGL API and provides vertex array objects (VAOs) which encapsulate vertex array states. These objects keep pointers to vertex data and provide names for different sets of vertex data.
The WEBGL_compressed_texture_atc extension is part of the WebGL API and exposes 3 ATC compressed texture formats. ATC is a proprietary compression algorithm for compressing textures on handheld devices.
The WEBGL_debug_renderer_info extension is part of the WebGL API and exposes two constants with information about the graphics driver for debugging purposes.
The WEBGL_draw_buffers extension is part of the WebGL API and enables a fragment shader to write to several textures, which is useful for deferred shading, for example.