NoEmotion HDRs might be the best place to look for free skyboxes.
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.