Moderndoorbells.com has a limited selection of modern doorbells, but the ones they have available take the cake for best-looking.
Doorbell Expressions has a much larger selection, including mailboxes and house numbers. However, some of their modern doorbells tend to look like spaceship launch buttons. I found this cool door chime on their website that looks like a bell. I did some digging and found that these doorbells are also available on a number of other websites.
Chiasso also has a nice-looking modern doorbell.