Normal day job: Professor of Mathematics at University of Washington (number theory). This is a tenured position, which gives me a lot of flexibility for "side projects". (http://wstein.org)

Side project: SageMathCloud, which is a collaborative web application to make teaching and research in mathematics using open source software easier and more accessible. (https://cloud.sagemath.com)

I plan to swap the two starting in June 2016, so that being a professor would then be my side project.