Beeminders
Beeminder is a service that allows you to set up “commitment contracts”: do $X$ or get charged $£Y$. I find these really useful for getting things done which I otherwise wouldn’t, like running, completing problem sheets or writing blog posts.
This page is a catalogue of some of the Beeminders I currently have set up. I’ve also written a blog post about how I use Beeminder for studying, see [[Beeminding as a university student]]B.