The RansomWhere? utility has recently come to my attention. This free utility by Objective-See attempts to thwart OS X ransomware by monitoring the file-system to detect if a suspicious process begins encrypting your files. If the utility detects that files are being encrypted, it will notify you and give you an opportunity to terminate the process.
RansomeWhere? only works by monitoring your Mac to detect when encryption is in process, so by the time it detects activity you’ve already been infected and some files have been encrypted. But the idea is that it will stop an attack early in the process before too much damage can be done.
The utility isn’t foolproof. It’s possible you may get some false positives and there are limitations. You can read the author’s blog post with more information and technical details about the limitations of the software. But it’s a good first step if you’re concerned about ransomeware and want to be proactive.