Check first, to see if it’s salable.
Vintage concert tees, ranging from the Who to the Stones, can provide a goodly amount, especially if they’re in good condition. Check Ebay, or do a general search. (Now don’t you regret eating that mustard dog in 1983 while you were watching U2 in the rain at Red Rocks?)
Turn it into a pillow. Sew the sleeves and bottom edge shut, then stuff with fiberfill.
Make a diaper cover for your favorite baby. (Tutorials for this and other links can be found below.)
Trim away the sleeves, cutting slantwise, for a new tank top look. Or cut away the sleeves, plus the upper back, leaving the neck intact. Voila – a hipster halter.
Cut out the design, using a square or rectangle. Trim it to size for use as a potholder – use the rest of the t-shirt for padding and backing. (Easy potholder instructions are here.)
Make a t-shirt quilt! It’s one of the most effective ways to preserve a whole drawer of memories…and clean out your wardrobe in the process. (Fabulous Tee Shirt Quilts by Caryl Schuetz is especially good – it’s got a range of designs and ideas.)
Want an even easier bedcover or snuggle quilt? Cut away the back of each t-shirt, taking care to trim just outside the side seams, so the t-shirt’s raw edge automatically folds under. Cut a sheet or waste piece of fabric to the size you want. Now position and stitch the t-shirts in place, sleeves and all, overlapping as you go. (The example I saw filled in any open areas with bright-colored socks. Use a mix of children’s and adults’ shirts for better contrast.) This can be top-stitched by machine, or whip-stitched by hand – your choice. Add an additional backing, then bind the edges.
Sort your t-shirts by color, cut them into strips and crochet a rug. A good basic pattern is here, from My Messy, Thrilling Life.
Line the bottom of a raised garden bed with it — then pile the dirt on top. Helps keep the weeds at bay while your shirt goes back to the earth. (Kind of fitting, considering how much dirt you rubbed into it over the years.)
Or failing all else:
Turn it into a soft polishing or dusting cloth.
You’ll find 100 different ways to use your t-shirt on this wonderful link. They sell all sorts of t-shirt styles at interesting prices.