Image-Based.Lighting is an in-browser IBL baking utility, built on top of my custom Direct3D 11 javascript library (d3d11.js).
The site supports several environment map formats: .HDR files, .DDS cubemaps or six equally-sized, square images.
Resulting maps (BRDF look up table, irradiance cube and specular cube with mips) can be saved directly as .DDS files.