What’s the most basic, profitable product you have ever built?

Yesterday I saw a tweet from Pieter Levels asking why people have been working on their projects for months without launching it.

The replies indicate that the main reasons are:

• Project grows in scope during development — “just one more thing and will launch it”.
• Makers not secure that the product will have an acceptance in the market.

I’m also a believer that we should launch a product doing only one thing. But doing it in the best possible way.

Have you ever built anything simple enough, not spending months to launch that was/is profitable?