Filezilla is a popular FTP client (and server) for Windows, OS X, Linux and other platforms. There’s no Chromebook client, but there are some alternatives you can use.
net2ftp is a web based FTP client. You can just head to the site, input the FTP details you need and start uploading / downloading at will, but bear in mind that you are sharing your username / password and activity with the net2FTP server. Alternatively, if you have a cloud server of your own, you can install net2ftp on it yourself and not share anything with anyone.
If you’re only looking to download files over FTP, Chrome itself can do this just fine. Just enter your FTP server’s details in the Omnibox like this:
… or, for anonymous login, enter details like this: