FileZilla should, in most cases, be the de facto standard free FTP client for Windows, Linux, and Mac. It's simple to use, supports FTP/SFTP/FTPS, offers a free FTP server solution, has IPv6 support, includes filename filters and directory comparison, logs to a file, has a tabbed interface (so you can have more than one FTP connection open at a time — even transfer files between servers), and much more. This is the cross-platform drag-and-drop FTP client you've been looking for.
Jack Wallen is an award-winning writer for TechRepublic and Linux.com. He’s an avid promoter of open source and the voice of The Android Expert. For more news about Jack Wallen, visit his website jackwallen.com.