FileZilla is no doubt a good ftp client and free as well.The opensource FileZilla 3 is written in C++ and uses wxWidgets as multi-platform toolkit so you can tweak according to your need.There are some shortcomings of this great ftp tool like "Failed to retrieve directory listing" error when viewing httpdocs on windows server file system.

I have switched over to another free and good alternative to FileZilla called CoreFTP , its good !, but not open source.However , It does not give the bold listed error as i mentioned above.

Download this free ftp client called Core Ftp  from http://www.coreftp.com/download.html