Mac only: Dropbox has a lot of great clever uses, but here's one we hadn't thought of. Frenzy uses your Dropbox storage as a private, feed-based social network for you and just a few of your friends.
I, like many of you, have a small group of friends with whom I communicate regularly outside of public channels like Facebook. We all live in different places, but we're able to stay in close contact by having long email discussions, sharing interesting links or photos with the group, and other such things. What's nice about email is it lets you keep the conversation private and uncluttered, and lets you share things more specific to the group that would otherwise clutter up your other friends' Facebook or Twitter feeds.
What Frenzy does is move all that to a Dropbox shared folder, using a small menu bar app that lets you view the shared folder as a social feed. People that are part of the shared folder can share files from Finder, photos from iPhoto, music from iTunes, or URLs from any browser with messages attached with everyone else. All the social networking out there can get overwhelming these days, but Frenzy makes it simple by constraining to just you and a few of your friends, which makes it really, really cool (and more user-friendly than trying to handle a huge email thread). Plus, it's a clever way to use Dropbox that we had never even thought of. Hit the link to check it out.
Frenzy is a free download for Mac only.