PhET is an awesome resource from the University of Colorado at Boulder. The site has interactive simulations for physics, chemistry, biology, earth science, and math that teachers and students can use.
The simulations can be run on the site or downloaded. The simulations are accurate and real and can help students learn concepts in more depth. They are easy and fun to use, and really do demonstrate the concepts well. In the physics ones I use, a big benefit is that students can quickly and easily change variables and conditions and then see what happens. This is very hard and time consuming in many hands on labs.
The simulations are also a great alternative to live labs if you don't have the resources available. Some of the simulations are better than any hands on lab because the students can truly see and understand what is happening.
You can search by topic, grade level, or just see the newest ones added. There are teacher resources for many of the simulations. Teachers who use the simulations post lesson plans and resources to the site for others to use also.
I use the Physics simulations in my classroom and they work great. This is one of those sites that every science and math teacher needs to check out.
Here's a list of some of the new simulations they have: