Part of the fun of DIY electronics projects is tinkering with things to learn how they work, but that doesn't mean planning ahead isn't a smarter idea. If you want to toy around with exactly how circuits would work, Lushprojects has a little web app for just that.
This project's a port from a Java application, but the fact it's built in HTML5 means it's accessible in every modern browser. With the web app, you can add in all sorts of components, wires, and whatever else you need to see exactly how a circuit will work. This way, you can save yourself some trouble when you're actually building these things in real life. Head over to the Lushprojects site to check it out alongside a video demo that walks you through the basics of using it.