At the time of writing, the latest version of OpenDarwin is 7.2.1.
You can download the latest installer from the downloads page.
When downloading the compressed ISO images from a browser, the browser sometimes recognizes the compressed data stream and decompresses it during download. This changes the checksum of the resulting file and does not necessarily mean the image is corrupt. If the mismatched checksum bothers you, we recommend downloading the image using curl, wget, or a dedicated ftp program.