Windows: Cyberduck, our favourite FTP application for the Mac, just released a Windows version complete with all the features familiar to Mac users: drag-and-drop file transfer, integration with other applications, Google Docs and Amazon S3 support, and more.
If you've been unhappy with the FTP clients available for Windows, you'll definitely want to try out the new public beta of Cyberduck. It's a simple to use yet powerful FTP client that works with any FTP or WebDAV server, as well as many popular services like Google Docs and Amazon S3. Furthermore, if you're already using an FTP client on Windows, Cyberduck will import your bookmarks and connection details into Cyberduck so you don't miss a beat. And, unlike many cross-platform pieces of software, it'll match the Mac version feature for feature from the start, so you don't need to wait for it to catch up.
Cyberduck is a free download for Windows.