Loryn: Conventional sewing wisdom says that shouldn’t sew with old thread, because it breaks too easily. This is one of those areas where I ignore conventional wisdom, and I think you should, too.
I see lots of vintage thread at garage sales and auctions, and I buy it whenever I can. The best part is that you can get a wide range of colors that are hard to find in modern thread. I haven’t had any trouble with old thread breaking, but I wouldn’t use it for parachutes or the crotch seam of your pants!
My favorite use for old thread is for topstitching, especially the small, brightly colored spools from the 30s to the 50s. You can find it in great colors, it’s usually 100% cotton (so it has great luster), and only using it for topstitching makes a small quantity go farther.
I use larger spools of thread from the 60s and later for general sewing. Just like with the older thread, you can find it in unusual colors to match vintage fabrics, and you can find lots of different weights. And you just can’t beat the price!