Ages ago, I wrote about my search for a file sharing application. I eventually went with BeInSync, and used it quite a bit over the past 18 months or so. However, they haven't updated it recently, and there are problems when editing Excel spreadsheets inside the shared folder (I told BeInSync about this problem, they fixed it, and then broke it in the last release). All in all, it was just showing it's age.
Armed with a new set of fairly light requirements (Mac/Windows support, permissions, auto-sync that doesn't fail), I evaluated the space again and came out with a very positive experience of DropBox. It works very well between Mac and Windows, has an awesome web interface, and hasn't exhibited any lags or slowness. In addition, it has the ability to revert to previous versions of documents, which is a nice feature that all developers have come to appreciate.
I'd like to see a couple features in the near future (native iPhone app with support for iWork documents, ability to set your own private key), but this thing was dead simple to install and use, and has worked brilliantly for the past month with a small group of people.