Crewhu -- They are arguably the biggest name in this space (at least in our industry) and offer a very fully featured platform for customer interaction as well as internal employee interaction.
Bizratings -- Also a pretty large name. We did not get to a trial with them.
Simplesat -- Very similar to SmileBack in its elegance and simplicity.
CustomerThermometer -- This is the vendor we ultimately chose to go to after being with SmileBack for several years. The monthly cost difference for our particular use case made too much sense. While not the most feature-complete tool, there are more options overall in addition to being much cheaper.