Apple's iCloud Photos is a popular option for Mac and iOS users, but I'm sticking with my folder based organization.

There are some good options for iOS when using Dropbox to manage your photos, but you are left with a sub-par viewing experiencing on the Mac, though. Folders are great for back up and organization, but it's enjoy your photos. Unbound is a great option, though. It simply reads the folder you point it to, and you get a great viewing experience. It doesn't create a separate library, so it's easy to sync to another Mac. Any edits that you make in another photo editing app are also instantly shown.

Get it on the App Store for $9.99.