I'm gonna wait for Steam version
Free work is ok if it's not-commercical project, such as modding for game. If it's commercical and they're going to make money off your work then it's unfair.
NoEmotion HDRs might be the best place to look for HDRs. It is basically a treasury of free HDRs, which you can use for both personal and commercial work. The developers spent 350 hours to create 150 HDRs. Images were taken over a period of roughly 10 months.
The collection includes day, evening and night scenarios. Most of the pictures are said to be taken around Prague and Czech Republic. A few mountain shots are from Swiss Alps. You can use these HDRs with any render engines.
Each image is 300MB and there is a bandwith limit of roughly 1TB per day, so please don’t download everything at once. You can find the breakdown of the creation process and other details on the collection here.