BitTorrentSync: Dropbox for Nerds
It's not really fair to compare Dropbox directly with BitTorrentSync. First of all, my title implies Dropbox is somehow inferior. To be honest, I haven't found anything that works as smoothly as Dropbox when it comes to sync reliability and ease of installation. That said, although it has incredible strengths, it also has a few shortcomings. One, it's not free, and if you have a lot of data, it can be expensive. Two, your data is stored on the Dropbox servers. The second one is a showstopper for many folks.
BitTorrentSync uses the BitTorrent protocol to keep multiple locations in sync. Everything is stored on your own machines, and you're limited only by your own storage limitations. The basic features of BitTorrentSync are free, but like Dropbox, there is a paid version. For me, quantity of storage is the most important feature with a syncing app, and with BitTorrentSync, that's unlimited. If you've been in the market for a Dropbox replacement, I urge you to check out BitTorrentSync (or more properly just "Sync" as it's been rebranded, probably due to the bad press "torrents" get). It's matured to the point that I trust it with my data, and its cross-platform nature makes it easy to distribute. Grab a copy at http://www.getsync.com.