If you ever wanted to try, test or create circuits without the hardware or equipment this resource is for you!! 123D Circuit is a drag and drop design environment that allows you to take different electronic parts, virtually wire them up, and see if your circuit works or not. It does require a free account, and you can create multiple circuits that you can also share with others.
If your library is interested in electronics programs, or even looking to get or have Arduino's this is a great supplementary tools for programs. You can have a class where you can teach basic circuitry design and principles without the expense of the hardware. Furthermore, if you have Arduinos and want to have patrons test their ideas before wiring up the board, this is also a great tool!
Even if you do not have circut boards, but would like to add electronics programming to your library, I highly recommend you give this resource a try.