Making extra thick stew
The method that I learned from an Irish co-worker is to add instant potato flakes.  You're basically making really runny mashed potatoes.  As stew is often thickened by the starch from the outside of whatever potatoes are in there, it won't throw off the flavor.

I've found that I can hold back a potato from the stew that I shred directly into the pot after everything else is cooked to the degree that I want, but while it's still near a simmer.  It will break down in minutes and thicken the stew.  You might need to try it to find out how much potato you need for the viscosity that you like.  

I suspect that the amount will change with the type of potato.  I typically use yellow (medium starch) potatoes.  I suspect that russet (starchy/floury) potatoes would work well, but that red (waxy) potatoes may require more and longer cooking to break down.