[morselsrule]

I would go with MySQL simply because so many big sites have used it: Facebook, Youtube, Digg, Yahoo, etc. If it's good enough for them, and free, it doesn't seem like you can go wrong.

See a list of companies using MySQL: http://www.mysql.com/news-and-events/press-release/release_2...

Or a talk from the guys at YouTube about how they scaled: http://video.google.com/videoplay?docid=-6304964351441328559

The only exception would be is if you are primarily handling financial transactions. Then perhaps Oracle would be the way to go, since your customers will be more conservative about the technologies they would accept.