Okay, I know it will sound like a Hallmark card but my best friend is my wife. When we met, we were friends for like 10 minutes and then... things developed further. Okay, it was about a month and a half between the first "meet for coffee and chat" and when she leaned over and kissed me. That will be 17 years ago in a couple months.
We've always kept that friendship, and built on it. It's made a really strong relationship. We love each other, we have a child, but the foundation is a really strong friendship. It's also the only friendship I've been able to maintain for this long in my life. I'm at a point now where I'm not interested in friendship really, and if something happened between us I'd be fine alone (well, semi-alone, we'd be cooperating on raising a child,) but I'm much happier with her than I'd be without her.
Other friendships I've had did not do this, so it's not every friendship for me, but everyone I've been in any sort of long-term relationship with has started as a friend.