Transmit 3 by Panic is by far the best FTP client I've used. I worked at an ISP for 8 years and manage several websites so I certainly have tried my share of them. Why is Transmit the best? First off it works - I've never had it fail on me in any way shape or form. It is easy to use and intuitive. Finally it has so many features I haven't even begun to scratch the surface of them in the year+ I've had it.
One of my favorite features is tabbed ftping - you can have multiple sites up at the same time and jump between them. There is also a dashboard widget and dock send features where you can just drag a file over the icon or widget and it will automatically upload. It is very mac like in its interface and you can search and even view in column view.
There are very regular updates that have added many new features. They have all installed while keeping my settings exactly the way they were. If you need a feature packed FTP client I highly recommend you check out Transmit!