At the moment I'm working on a project that has a lot of downtime. It takes 10 minutes to load the development environment, builds can take half an hour, loading the application for debugging sometimes takes 15 minutes. I would like to know what sort of productive work (besides contributing to StackOverflow) people do during these times when no "real" work can feasibly be done. I usually find that any sort of work I do during these times takes my mind off the task at hand and it takes me a while to get back on track after the wait is over, which means that I might have been better off just twiddling my thumbs while waiting.
Sorry if this question is a bit too subjective, but it is my main problem where I work. And I do know that these time sinks should ideally be eliminated, but there will always be some sort of downtime to contend with.