This morning I stumbled across a review of Synkron, an open-source synchronization application for syncing folders on different machines, or simply syncing folders on different hard drives. Available for free (as in cost and as in freedom/development) on Sourceforge, Synkron is cross-platform, easy-to-use, and powerful.
The program sports a tabbed interface, making it easy to set up, run, and manage multiple jobs. The interface is uncluttered and easy to understand. Folder analysis is extremely fast, and files, file types, and folders can be blacklisted to exclude them from synchronizations.
Of course, you don't have to go open source to get good folder synchronization. As Walt Mossberg's review of SugarSync suggests, there are excellent proprietary options available, often for little or no charge.
But if you want the flexibility of being able to highly customize your folder sync solution, Synkron may be for you. Give it a try.
Here's what it looks like: