The internet has some wonderful physics puzzle collections.Here is an excellent one on cosmology: universeinproblems.com.
The internet is small; free physics books are found in it as frequently as birds are found in aquaria. Here are the best ones:
• Friedrich Herrmann's excellent, short and freely downloadable physics textbook for secondary school, The Karlsruhe Physics Course, can be downloaded at www.physikdidaktik.uni-karlsruhe.de/index_en.html. It is available in several languages. This book is also recommended if you – girl or boy – need to repeat what you learned or should have learned about physics, the science of motion, before the age of 18. If you are a professional physicist and are interested in the background of this book, you might also read the 2013 paper by C. Strunk and K. Rincke.
• Two excellent introductory physics texts by Benjamin Crowell, Simple Nature and Light and Matter, as well as a textbook on calculus and a textbook on general relativity can be downloaded at www.lightandmatter.com.
• The Feynman Lectures on Physics, now over 50 years old, are online at http://www.feynmanlectures.caltech.edu/info/.
• For learning physics for university examinations, the site of reference is Rod Nave's Hyperphysics at hyperphysics.phy-astr.gsu.edu/hbase/hframe.html.
• The high-quality electrodynamics text by Bo Thidé, Electromagnetic Field Theory, can be found around the internet.
• An introduction to quantum mechanics that uses Schwinger's approach, Quantum Principles and Particles by Walter Wilcox, can be downloaded at blogs.baylor.edu/open_text/purpose/quantum-mechanics/.
• Pearls for every physicist are the fallacies about physical concepts collected by Friedrich Herrmann, Historical Burdens on Physics, downloadable (in German) at http://www.physikdidaktik.uni-karlsruhe.de/Kolumne/Altlasten.html. This collection of short texts is a treasure chest; it makes one think and enjoy.
• If you want to explore physics in a completely different way, have a look at www.numericana.com.
For an extensive list of internet links on physics see Appendix C: Sources of information on motion in volume I of the Motion Mountain physics text.
To learn physics, two wonderful printed and electronic textbooks about motion are by Rainer Müller: Klassische Mechanik - vom Weitsprung zum Marsflug (see here) and Thermodynamik - vom Tautropfen zum Solarkraftwerk (see here). They are the best books on these topics that you will find.
To do physics experiments simply with your smartphone, go to www.phyphox.org and download the free app.