FileZilla

FileZilla is a fast and reliable FTP client with lots of useful features and an intuitive interface.

Main Features:
- Ability to resume Uploads/Downloads (if the server supports it)
- Custom Commands - Site Manager with folders
- Keep Alive system
- Timeout detection
- Firewall support
- SOCKS4/5 and HTTP1.1 Proxy support
- SSL secured connections
- SFTP support - Upload/Download Queue
- Drag&Drop
- Multi-language support
- GSS authentication and encryption using Kerberos

There also is a server component available at the author's site.