RansomFree is a free tool which monitors your PC looking for ransomware-like activity, immediately suspending the offending process just as soon as encryption starts.
RansomFree works in part by creating dummy images, Word documents and other media in various locations on your hard drive (we had a couple of randomly-named folders in our Documents folder). Presumably the hope is that if ransomware strikes, it hits these files first, giving time for the program to detect the threat without you losing any data of your own.
Developer Cybereason says the algorithm is based on analysing the behaviour of more than 40 ransomware strains, and because detection is based on actions, not signatures, it's entirely possible it'll block many more.
Setup couldn't be easier. There's nothing to do or configure, just install the program and it runs in the background. The only operational setting is a "Pause for one hour" feature, maybe handy if it conflicts with some other software.
Resource use is limited, too. Our test system gained two background processes which used under 35MB of RAM in total, and a regular but very small amount of CPU and hard drive time (a fraction of a percent).