When I started this site in 2008, it was very difficult to find resources that were easy to use in the classroom. Things have got a lot better. What follows is a list of resources, organized roughly in order of how useful, easy to access and easy are they are to use in the classroom.
1) Perimeter Institute for Theoretical Physics (PI): I believe that this site has the best resources for teaching modern physics to high school students. (I worked on many of these, so I may be biased.) The material is presented at the appropropriate level with strong, subversive curriculum links. The packages contain lots of support for the teachers; worksheets, worksheet answers and extra background information. They promote student-centered learning with hands-on activities, Peer Instruction etc. Unfortunately, the materials are not available on-line. They are free to Canadians, but all other teachers need to pay around $25 per kit.
2) Physics for the 21st Century This is a HUGE project. There are eleven half-hour videos (which break easily into twenty-two fifteen-minute videos) which are great examples of current research efforts. Each video comes with on-line information and activities. It was designed to be a course for high school teachers who want to develop their understanding of the latest developments in physics. These lessons can be modified quite easily for a high school class. The site can also be used by very keen independent students. It covers the full range of modern physics with an unusual emphasis on Bose-Enstein Condensates. It is completely free.
3) Physics Education Technology (PhET): This site contains many, many simulations. The modern physics ones deal with quantum and atomic physics. The simulations allow students to explore topics that would otherwise be impossible or too abstract. This is not a good place for a teacher to start learning about the physics, but it is a fabulous place to deepen your knowledge and to build a lesson with lots of PEOE questions etc. There are suggested lessons to go with the simulations, but I haven't found them all that useful. Most provide questions - but not the answers and not extra background knowledge that a teacher might need.
4) Tri-University Meson Facility (TRIUMF): This site has two great resources with videos, activities and extra information for the teacher. One uses the cyclotron to provide data to do an experiment to establish the formula for relative momentum and the other shows how electric and magnetic fields are used to acclerate, guide and measure the mass of particles.
5) Fermilab This is a large and frustrating site. However, there are a few great resourcesin particle physics that you should check out.
6) LIGO The Laser Interferemometer Gravitational-Wave Observatory (LIGO) has some very good teaching resources for senior high school which you can find at Einstein's Messengers. There is also a 20-minute documentary which you could play for students from grade 9 to 12.
7) Cornell has a focus on nanotechnology and a suite of labs that can be borrowed for free but only if you have attended one of their courses. This great for teachers who can attend one of these. The resources look really good and I found one great activity that I could put together myself with the use of two pencils and an elastic band to make a variable slit for diffraction of light. They also have a kit for measuring Planck's Constant, but you can put that together easily for yorself. There may be others, but I haven't had the chance to dig through these thoroughly.
8)Minute Physics : Henry Reich has put together a fabulous collection of over seventy short videos that mostly look at modern physics. The videos are all hip and snappy and they make great use of animations and humour. They are a wonderful tool to stimulate general interest in physics. They can also be used to promote learning, but they go really fast and they need to be integrated with other resources and activities.
9) Einstein Online This is a great site to learn more about relativity - for teachers or very interested students. I especially like the way they have a section called Elementary Einstein which is a good place to start. Other sections go into as much depth as you can handle. I especially like The dark heart of the Milky Way.
10) Sixty Symbols This is a huge collection of way more than 60 short videos from the University of Nottingham. They usually involve modern physics . There are some great demonstrations but there is usually too much casual chatting without visual support. I like to show E = mcc and have the students listen for what the short version of this equation misses. I also like Polarization as it shows polarization with microwaves quite nicely. When I get time I will try going through them to find the good ones. If you have already done so, please let me know! They can make great additions to lessons.
11) CERN This is a huge and very frustrating site. There is alot here - but not much that is useful.