Core FTP is an FTP client that comes in both a free and professional version. I'm not really sure who wouldn't find their needs met by the free version, but I'm sure there are some power FTP users who would find the pay version necessary. The main reason I can see is that with the Pro version you can schedule FTP transfers for a later time if you want to queue up some 3:00 AM transfers but still want to be in bed at a regular time. But, for most users Core FTP LE is a fine choice.
Core FTP supports all the security protocols out there including SFTP and SSL as well as HTTPS. Core FTP has several multi-pane views to choose from, so it can fit your needs regardless of your monitor configuration. There are two things about Core FTP that make it one of the greats. First, with Core FTP you can actually open two remote sessions and then manipulate files between the sites. In other words, you can drag and drop files from FTP Site 1 to FTP Site 2 without the step of downloading them to your PC first. The second thing about Core FTP is that it seems to be the client that will work when the others won't. I have never seen anyone explain why, but if for some reason your FTP connection keeps timing out or disconnecting with other clients, there is a decent chance that it will be stable with Core FTP.
Get Core FTP LE for free here.