MIT, Harvard, and many other prestigious universities from around the world have begun working together to provide high-quality education to anyone who wants it, for free. This is a phenomenal opportunity for professional development for substitute teachers.
One of the advantages of being a substitute is the chance to bring a new perspective or new experiences to the classroom. One of the great challenges in being a substitute (particularly at higher grade levels) is the expectation that you will be able to be knowledgeable in any area of study where you are asked to serve.
Websites like EdX.org and Coursera.org can be a great resource to help substitute teachers meet these challenges. Here, anyone can sign up for free and take courses in subjects ranging from ancient Chinese history, to hypersonic flight, to how businesses use the data that the internet constantly collects on us. There are even courses in education.
All of the courses offered on these sites are pass/fail, and will provide you with a certificate of completion if you make a passing grade. These can be converted into a transcript of your continuing education, demonstrating to potential employers your dedication to continuous self-improvement. The lessons learned in these courses can serve as emergency lesson plans for various subjects, and will set you apart as a having unique value that can be brought to the education of the students at this school.