Many editing programs have a slow motion function as an add-on, but this is often very limited, and the user quickly realises what its limitations are. Anyone who wants high quality yet not too costly slow motion or time-lapse should work with proDAD's ReSpeedr.
With ReSpeedr, you can "Pretty easily improve videos with detailed slow motion and fluid time-lapse. The possibilities are endless," according to the field report by flatflutedivers.
One use is to better capture the rapid passage of a rare fish or schools under water with the slow motion effect of ReSpeedr and so literally immerse the viewer of the video in a whole new world.
Here is a review of the underwater films post-edited with ReSpeedr: