You could pretty much consider FileZilla an open source success story. It's popular, works well and fast, and regularly updated.

If you're looking for a solid FTP client what more could you ask more? In fact, FileZilla is not just limited to FTP, but also handles FTPS and SFTP for secure transfers of multiple files on multiple servers. The interface, if not a little ugly, gives you a quick outlook of all your transfers, sites and disks and makes it easy to drag and transfer. I also quite like the QuickConnect feature to access your favorite server in seconds.

FileZilla is known to be fairly fast, and can also resume downloads and handle very big transfers, I'm talking here higher than 4 GB. You can set both local and remote filters for things like images, explorer files or CVS and SVN directories or even create your own. FileZilla also includes a tool to compare directories in terms of file size or modification time.

The application can guide you when setting up your network and lets you set speed limits in case you don't want to overload your bandwidth.

One thing really missing from FileZilla though is a scheduler to automatically set transfer times.

Despite lacking a scheduler, FileZilla is an excellent open source FTP client. It's stable, fast and very efficient. Highly recommended.