A website I have been using with increasing fervor is PhET, which has been developed by the University of Colorado. It is full of well organized, research based science simulations. My students find the simulations fun, and when I move around the room to listen to their reactions, I have observed a great amount of higher order thinking.
The last simulation I used was the one titled Density. I began my lesson by explaining briefly what mass, volume, and density are and how they relate to one another. I then showed my students the simulation online and explained a few of the things they could explore. Next, I divided my students into groups of two or three and told them to go play with the simulation and talk to each other about what they were observing. We regrouped at the end of class to discuss what they noticed. Their responses were incredible, and they were disappointed that science class was over already. I followed up the lesson by assigning students to work with their partners to find out what the mystery containers held. The students turned into science sleuths as they measured mass and volume, calculated density, and identified the correct objects on the density table.
Overall, I have been extremely impressed with all the simulations I have utilized from PhET. I myself learn a lot form interacting with them. Some things I feel need to be hands on in science, but other topics, especially on the molecular level are difficult to teach to a fifth grader. This is when I find simulations especially useful.