I Know that Classical Music Tune - Just not Where it is From

I was watching television the other night and in a late scene the sounds of the opening movement of Bach’s Cello Suite #1 came forth. I smiled in recognition. And then thought, this is a tune that gets used over and over again and I suspect there are many who recognize it but don’t know its origin.

Are there other classical music snippets that you can think of that are similar in being immediately recognized but its source unknown? (Skip Beethoven’s 5th for example.)

One that I thought of is Wagner’s "The Ride of the Valkyries”. The tune is associated locally with our WNBA basketball team – the Golden State Valkyries – but how many know it is also from an opera.
Others?