One thing I wish I did more often is checks to see if I understand the world - Fermis, landmark numbers, predictions about how things work, etc.
Along the way I started wishing I could cover up numbers on websites so I would have the chance to guess first and then reveal via mouseover, and Aur Saraf agreed to make a chrome extension, and here it is! Lets you set the default for a whole site or a page. (Not good for dynamic sites like twitter)
The code is open source here if you want to add any feature requests.
(I think I want to change the name - let me know if you have thoughts)