Realistic character emotions Blender has a wide suite of character making tools, but I have been having trouble finding an emotion-to-facial-expression "conversion tool".

Make Human is very realistic and has a wide variety of options that control genetic facial traits. However, emotions have to be manually encoded in the riggings (this is my understanding at least). Doing so in a convincing way isn't trivial.

Is there any tool that "renders" emotions? Namely, you input emotional descriptions or parameters of some sort and it will adjust the rigging of a make-human mesh or a even just it's own stock character? 

Ideally such a tool would allow complex mixtures of emotions and animations. For example it would be amazing if you could specify “character is startled (a 6 on a scale of 1 to 10) from an event that occurs at a relative azimuth of 30 degrees. The startle shifts to grief over a period of [a default value that reduces the guesswork, but can be changed].” This is probably pipe-dreaming, but even a basic tool would be very helpful.
Try Blender's Pose Library. If you create a pose you like manually, you can select the bones that correspond with it and press the "+" button in the Pose Library to create a preset. Now when you want that emotion, all you have to do is go in (select your bones first) and click the magnifying glass icon to assign the pose to a given frame.

Additionally, assuming your various characters use a standardized bone layout/naming system, you should be able to use one library with multiple characters with minimal tweaks.

Please note this does require a lot of manual work on your part to set up initially, so if that defeats the purpose of your question, try looking around for preexisting facial expression pose libraries online. (I would recommend making your own, though, as a third-party library may not use the exact bone/rig naming setup and cause some major issues.)