I loved this idea found at the Liz Marie Blog so I decided to try it and I'm in love.
I found my window at the Resource Yard in Boulder.
Window: Free pile
Wall clings: $4.99 on clearance at Joanne's Fabrics
The tutorial is very simple:
1. Find old window
2. Clean old window
3. Peel and stick wall clings
5. Admire your handy work
If you'd like more info or another example, definitely check out the breakdown from Liz Marie.