What's Your Favorite Business Tool?

There are business tools that owners use because they feel they have little choice. Either there’s nothing else, or the pain of switching seems too great. But are there tools that you actually look forward to using? Which one have been most helpful in building your business, and what have you learned about using them that wasn’t immediately obvious?

If someone recommends a tool that hasn’t worked for you, feel free to ask the commenter how they solved whatever problem you had.

And, of course, if you’d like, you can also tell us which tools you’d never use again.