Windows: Syncless is a file synchroniser that offers not only traditional folder-based synchronisation but also tag-based synchronisation for easy file syncing.
You won't find a shortage of file syncing applications but Syncless is the first we've reviewed that supports tag-based synchronisation. Rather than simply plugging in folder X and folder Y and telling Syncless to keep them in sync — although you're free to do so — Syncless groups tagged items together so you can easily tag multiple folders as "Photos" and sync them all to each other. Check out the video below to see it in action — the video is a release behind, but it showcases the application nicely:
Syncless is freeware, Windows only. Have a favourite application for keeping your files in sync? Let's hear about it in the comments.