CircuitLab is an electronics simulator which you can run in a browser. Above you can see one of the example circuits provided to help show the power of the application. You can build your schematic (perhaps you want to try [Jeri's] psu shut-off timer?) in the editor mode, then switch over to the simulator to get data back from the components. In that mode, your cursor becomes a probe, and clicking on different parts of the circuit will return the calculated input and output voltages for that component. But wait, there’s more. It’s got time and frequency simulation in addition to the voltage simulator. This lets you look at waveforms fed through analog filters, or timing data like in the 555 timer circuit above.
Where does this fantastic tool come from? [Humberto Evans] and [Mike Robbins], the guys behind NerdKits developed this site. We’ve seen a lot of their hacks around here, like milling solenoids and making them play a xylophone. Check out the CircuitLab quick start video they put together after the break.