Harvard University just made its 2009 photography course free to access online. The content is a little dated, but it is free, and still a great resource for leveling up your photography skills.
Available on Alison, the course has 12 modules covering the fundamentals of photography – from exposure, optics, colour and histograms to artifacts and software tools.
Complete the assessment at the end and you'll score yourself a certificate. You need to get a mark of 80 per cent or higher to pass.