I built Swiss Army Tabs to recover my (frequently) lost Safari windows. But I kept adding features to it (multiple browsers, fixing expired Google searches, navigation). Then a Google engineer said he'd only be interested if it could search history going way back in time. I ended up building a search with natural language input in 29 languages, and natural language output also in 29 languages. I enjoy just playing with it. And it has restored my Safari tabs multiple times already.