Like doors, old windows are great for upcycling, there's just so much you can do with them. For example, you don't even need to add new panes in order to turn them into great-looking picture frames. They are also great for adding a bit of a farmhouse-inspired look to your home. If you like, you can create an entire focal point around them as this person did.
They also look good above mantels, and if you aren't in the picture frame look, then consider having a few mirrors cut, and replacing the old panes with them. This gives you a great mirror with an even better patina.