Always popular this post on Mathematics Posters is kept up to date. The post includes several excellent sources of free posters to decorate your walls and get your students thinking too!
There are of course endless possibilities for putting things on walls! Student work obviously but also definitions and key facts to remember particularly now in the days of no formula sheets! (See also Mathscard from Loughborough University) Or perhaps some exam terminology. Maybe put some mathematical puzzles on the wall for idle moments!