All operating systems come with a built-in FTP client that runs from a command prompt, using the command
You might want to check out the following Wikipedia links for more information:
There are also other fully-featured FTP clients available for various operating systems that support some extra options.
Below is a list of some of these third-party clients.
Mac OS X
Web browser FTP support
Additionally, all modern web browsers (Internet Explorer, Firefox, Opera, Chrome, and so on) include built-in FTP support as well that will do in a pinch. However, you would only be able to download content through the browser.
DreamHost WebFTP support
You can also connect to your site via WebFTP. This isn't intended to be a fully functioning FTP client, but will suffice when you don't have one handy. Uploads are limited to 7MB, but you may find it very useful for features like easily moving files and editing live pages.