Core FTP Lite is a top-notch FTP client with a wide, uncluttered layout to keep things organized. Easy to use and lacking only the most advanced of features--like multisite connections--Core FTP is a must-have.
Major functions, such as Site Manager for maintaining connection information, Quick Reconnect, and mode switching, are given big buttons on the toolbar. When you're connected to a site, though, the file-management pane also replicates buttons for most of these functions so you don't have to mouse very far to use the tool you need. The client supports all essential FTP functions, including multiple transfers, secure protocols, configurable ASCII uploading for specified file types, browser integration, and more.
There are a few downsides to using this app: there's no scheduler; some users have noted past stability issues, both in general and when conducting large transfers, but it seems that many older bugs have been worked out. Strongly recommended.