For me it's a todo list. If I'm playing a game, researching a topic, and so on I have a window with tabs in it for that one thing. This works well enough and it feels nice shutting the window for closure when I no longer need it.

The problem is when I have stuff that sits around for longer like the Fast AI course I'm doing. That window is taunting and mocking me.

Edit: Looks like I have this in a tab because I meant to finish reading it. 7 months. I have a problem.

https://news.ycombinator.com/item?id=24737026