WinSCP which also handles command line copy and paste of files now. I can also click edit and do whatever editing I need of most files. (html, php, css, xml are all I've tried so far). I can sit and have it open for hours at a time and not have it log me out, unlike 99.9% of ftp clients.
I was moving files this weekend from one domain to another on the same server. All I did was copy them in the command line on WinSCP and paste them right back to the new location without having to download them and then upload them again.
I have a paid FTP (Cute FTP Pro) and several free FTP and NONE come close to the quality of WinSCP, esp since I am using a Vista computer. Even the paid ftp client which is suppose to be updated and compatible with Vista gives me transfer errors. I'm literally afraid to use it on a site. But I will use WinSCP all day long.