With the recent posts on successful/profitable side projects, I figure it'd be good to learn about some failed ones. Let's stick to "launched" projects instead of those that never happened.
What was the project and why did it fail (best to your knowledge)? Or what's a side-project of yours that's actively losing money?
If you read the financials from most software companies that are public you'll see that marketing costs are way higher than development and maintenance costs. It's the primary way to get customers to buy or use your app.