iPhone: Apple's Photo Stream, which automatically backs up and syncs your last 1000 photos for up to 30 days, is a boon for iPhone photographers, but Amazon just rolled out an app that tops it with lots of free storage.
The idea is pretty much identical to Dropbox's Camera Uploads feature on iOS, but with a few distinct advantages. You can set it to automatically back up your entire camera roll if you want, but you can also select only the photos you want for batch uploading if you prefer. The settings screen also offers a clever "Large Uploads" feature which prevents your iPhone from locking its screen, letting you keep the phone in your pocket without worrying about running into iOS's background uploading time limit. Unfortunately, the app will only import photos, not videos, which is a bit of a bummer.
Amazon gives you 5 GB for free with no time limit, which should hold more photos than the 1000 that Photo Stream allows. If you want more space, upgrades are very reasonably priced at $US10 per year for 20 GB, or $US50 per year for 100 GB, which is half the price of Dropbox's 100 GB plan. Like Dropbox, you can also access your photos through Amazon's web interface or its free Cloud Drive desktop apps. I wouldn't say Amazon's photo syncing solution is any better or worse on the whole compared to Dropbox, but it's nice to have on your phone if you don't want to waste precious Dropbox space with all of your photos.