Hyperfocal have released a large pack of high resolution (i.e. 10,000 by 5,000 pixel) skies, each with up to 16 steps of dynamic range, varying from underexposed to seriously overexposed. Each image costs around $59, and are designed for use within 3D environments, where they provide realistic reflections for semi-opaque surfaces, such as tinted glass, or atmospheric effects such as dust and smoke. This is beyond the scope of most small-scale designers, of course, but if you are desperate for high-quality HDR maps, you can't go wrong with this collection!