A good (currently the best) FTP client
This is one of the very few times I recommend purchasing additional software to accomplish a task in os X.
OS X has command line ftp and ssh built in. Ssh access to mediatemple provides some very powerful capabilities. And there are many free or shared alternatives for basic ftp.
But for heavy ftp use, like deeply nested file structures, synchronization and making powerful features easy to use, there is nothing that compares to transmit.
Just like a macbook pro costs more money, but provides features that make you glad you paid the extra, transmit provides value far in excess of the purchase price. Basically every time you have to use it, you will recoup your investment.
