Cover an top hat! (smile - I am sure everyone needs one!)
I have also pinned on Pinterest many other fun ideas for using old t-shirts. You can see them on my neat repurposing ideas board.
I had taken this picture of my niece wearing a vest that she had made from an old t-shirt specifically because I had a post like this in mind - then I forgot at first to include it. Here is yet another idea of what you can do with an old t-shirt.
I am also linking to Clever Chicks Blog Hop