How I Use It
PHET has different type of activities and simulations, but I´ve only used the one about electrical circuits. Easy to set-up as students just need to download the file from their site and run it (if your students use Mac, maybe they need to change the privacy settings to allow students to install it). Basically the PHET electrical circuits activity allows students to build their own circuits adding modifying different parameters and components. Students enjoy the idea of making their own ones and trying crazy things, especially changing the voltage and ruining the circuits... but at least they get an idea on what is going on and can apply some concepts previously learned. It´s a good activity if combined with some questions, as students may try few circuits and after 10-15 minutes get bored.
I like this site and some of the activities/simulation in there, but they have to be linked with your curriculum and maybe an activity prepared by you. Students have the option to explore and invent their own circuits with different components, and that´s great for creativity and also to put knowledge in practice, as if the circuit is not built properly, it won´t work thus also good for problem solving. You can use it for a lesson or for students to practice circuits at home (in case you don´t have those materials in the lab).