Free and engaging are two words to best describe PhET's collection of math and science simulations. They all promote inquiry and hone students scientific skills. Some are virtual labs and some are simulations that have a game like quality.
Labster is an award winning website that provides fully interactive, highly advanced lab experiences and simulations. It is not cheap but they provide a free trial. The topics covered in labs are geared toward advanced high school and college, however they do have some labs that could be used in all high school science classes.
Light, electricity, and magnetism can be really difficult concepts to teach elementary students, especially in a hands-on way. This site has 16 "web interactive" lessons that allow students to learn and do experiments and compare data!
The Biology Corner appears to be an amateur outdated website at first glance, but it contains some of the best and most useful resources for any science teacher. This is a link to a list of virtual labs created by Glencoe. Labs are divided into physical, earth, and life science.
GIZMOS is a subscription based site that has over 400 math and science simulations for students in grades 6-12. The topics covered in the simulations spans every topic in secondary science. They are CCSS aligned and come with worksheets and lab handouts. The simulations are engaging for students and can be modified for students who need it. GIZMOS now has an area where teachers can share and upload any handouts or resources they made to go with the simulations-no need to reinvent the wheel!
Scholastic's administrator blog has this informative article that weighs the pros and cons of virtual labs. It is a good read with reputable links and sources quoted. It also gives an honest account of what really needs to happen to make virtual labs work. For example, they will not work without any professional development for teachers!