Simulate HDR is a plugin for The GIMP that creates two layers from a single image simulating an underexposed and a overexposed shots, creating a High Dynamic Range image.
It can create a tone mapping layer to compress the color range of the image.
HDRFlow is a framework to process high-dynamic range (HDR) and RAW images. It's written in C++, and is both cross-platform and hardware accelerated on modern GPUs.