An FTP client helps you get your Web pages from your hard drive to your Web hosting server. A good one can do things like automatically update your site, create mirrored sites, and even allow you to make quick changes directly on the server. These offer many different features: from drag-and-drop to low-cost or free options. Once you've moved beyond a basic site, a good FTP client is a must.
Fetch is the most popular FTP client for the Macintosh. It offers the same familiar interface as Mac OS making it easy to get started. Fetch 4.0 offers advanced features like mirroring, account management, and integration with BBEdit. It also runs more efficiently on Mac OS X. If you're looking for an inexpensive FTP client for your Mac, the little dog (Fetch) is a great choice.
Transmit is a great FTP client for Macintosh. It offers features like drag-n-drop, Mac OS interface, multi-tasking capabilities, interrupted transfers can be resumed, and more. It is programmed to make it faster if you're on a slower modem, especially when you have a lot of files to move. I especially like the "site redial" feature - it tries sites over and over until you can get in to download.
Interarchy offers all the features you expect from a high-class FTP client. It includes features like mirroring, folder transfers, scripting, setting file permissions, and more. It also includes features of use to web developers, like editing files remotely, DNS and ping for checking access and server availability, and the ability to parse web pages to present links in a Finder-style interface.
iSynch is not precisely an FTP client, but it is a valuable tool for a web developer. It is a program that compares files on your hard drive to the files on your website. Any that are out of date are transferred to the appropriate place. I especially like the fact that it only uploads or downloads the files that have changed, thus making updates to a website nearly painless.