What to do with the fat rendered when making beef stock?
Rendered beef fat can be used in a lot of ways.  You can use it in place of oil in a lot of recipes, but finding out which ones you like will take some experimenting.  Around our house, I use rendered fat from beef or bacon in place of oil when sautéing, for example with onions and peppers, garlic or mushrooms.  I've also used it to add some kick to gravies.

You could use it to pop popcorn, which I've heard is delicious but unfortunately have never tried.

I've had pasties made with beef fat, and they were delicious.  Mashed potatoes as well.

As far as fat ratio goes, it's better than butter, but not as good as other fats.  Beef fat has a high smoke point and is suitable for frying.  

The table below is based off of 1 tablespoon.  Ratio means saturated to unsaturated.  Smoke point can vary depending on a lot of factors (olive oil can range from 300 when unrefined, to 375-450 when refined depending on quality) but the table below should be a good guide.  For the oils, I took the refined numbers.  


                   sat   mono  poly   ratio  smoke
  Canola Oil       0.9   8.2   4.1    1:12   470&deg;F
  Olive Oil        1.8   10.0  1.2    2:11   450&deg;F
  Chicken Fat      3.8   5.7   2.6    1:2    375&deg;F
  Duck Fat         4.3   6.3   1.7    1:2    375&deg;F
  Lard (pork fat)  5.0   5.8   1.4    5:7    365&deg;F
  Beef Tallow      6.4   5.4   0.5    1:1    400&deg;F
  Butter           7.2   3.3   0.5    7:4    350&deg;F


(fat source) (smoke point source)

A lot of people mix it into their dog's food, or use it to feed birds.

Seach for tallow if you want to find recipes that specifically use it.