This is a great article on the 37 Signals blog titled "Question Your Work". Jason Fried always makes us challenge ourselves with thought provoking ideas. In this article he asks some really great questions about why we do anything and what drives us to make effective decisions:

"Why are we doing this?"

"What problem are we solving?"

"Is this actually useful?"

"Are we adding value?"

"Is there an easier way?"

"What's opportunity cost?"

"Is it really worth it?"