Written in Django, got some HN love a while ago - no signups for a while now but we have over 500 accounts that signed up at some point. About 20 sprang for the "Pro" version at $10/month. We even had a customer pay $50/month for an API version integrated directly into their site.

It's used to have your customers send their browser details directly to your inbox. Just point them at your site such as http://example.browser-details.com (CNAMEs with custom domain possible).

Created to scratch our own itch: previously, most of our customer would be hard-pressed to name their browser version so it was very helpful in development. Uses an external API to map the user agent string to browser make/version.

Interesting trades considered. ;-)