Building electronics requires preparation, and that means taking a look at your circuit design before you start. Circuits.io allows you to do that from pretty much any browser out there.
Using Circuits is as easy as picking which circuits and PCBs you want to use, and laying them out on a grid. The webapp will let you know when your connections are faulty. As a simple, lightweight planner for your DIY prototypes, Circuits is handy to have around. For something with a little more weight, you might also want to check out DoElectronics or CircuitLab.