The best use of all for an old tee shirt is gun cleaning. Once I’ve oiled the outside of one of my guns, I wipe it off with an old tee shirt. I keep the shirt, never wash it, and use it over and over again. The mellowing combination of gun oil and Hoppe’s No. 9 works wonders on the guns.
Also good for shoe polishing. Same deal, let the polish saturate the rag; eventually you'll only have to open the polish can about every third shine. I have one I've been using about nine years now; well seasoned!