OK, I've edumacated myself. It turns out that White Labs has a product "Servomyces" which is a yeast nutrient, basically derived from dead yeast hulls. I believe it is more commonly used in wine where the must lacks the nutrients commonly found in beer worts. If you've gotten to a reasonable FG, then your yeast did fine without the extra help.
FWIW, I generally use Di-ammonium phosphate (DAP), though on the occasions where I forgot to add it, I didn't notice any problems.