On a friend's suggestion, I had a peek at Dropbox for syncing directories between multiple computers, sharing files with people without posting email attachments, and the like. It's got many rather useful properties, but seems a little immature in the unix world; it is unaware of file modes and symlinks (so symlinks will show up as real files or as nothing at all, if orphaned), making it less suitable for syncing git checkouts across multiple machines, as I was hoping to. As noted in the linked thread, though, the upcoming 0.8 release will get aware of file modes, which is a good start.
What it does seem really good for, though, is auto-syncing preference files, data sets of stuff you want comfy access to wherever you are, breaking through NATs to sync stuff to home machines behind firewalls, and the like. One of the neater ideas I came across in the otherwise mostly uninteresting comments on this tips and tricks post was to set up a home machine to poll for torrent files dropped into some Dropbox-synced directory and start downloading them (to another directory, presumably), instead of doing the same via ssh.
The free plan covers 2GB data stored (plus keeping 30 days of backup history), and if you sign up through a referral link (here's mine), both signee and referrer get a quarter-gig extra quota.
Their iPhone application delivers browsability of the files you sync (your own and those others share with you -- and it should be noted that "sharing" currently implements read/write access, only, so you'll want to keep backups and/or trust sharees as you trust yourself) and lets you micromanage pictures into your on-phone picture album one by one, lets you play music and video, but not add them to your on-phone music library.
Similarly, it lets you can micromanage a picture at a time back from the device photo library to cloud storage (and connected computers), or make (now read-only), copy and email urls to any of the files in your dropbox, instead of mailing them as large attachments.
I am not surprised that it doesn't much address the main pain points of the major data interop inconvenience that is the iPhone (App store terms probably don't allow them to), but I was more than a little surprised that it doesn't measure up to what a normal rsync does yet for typical machine-to-machine file transfers yet. It does a good job as a dual-direction sync feature for basic data between yourself and non-technical friends and relatives, though.