Ask HN: Have you ever created a private app with commercial appeal?

I've created apps (rss client, online bookmarking) for personal use, but I didn't bother making them into commercial products since I worried they would get just enough users for me to feel obligated to support them, but not enough to make any meaningful profit.

Has anyone else created a nifty app with commercial appeal? Do you sometimes feel like sharing it with the world even though you know it will be more trouble than it's worth?

Please share your stories.

Note: You don't need to share the URL--you can just describe the app.