When dealing with file transferring a simple and stable GUI interface is great. There are several FTP clients out there but the ones I am gonna list here are free and just as good as software that you can purchase.

MAC

1. Transmit
   The truck, by far is the best one for Mac users. It includes dual windows and various setting options, including live editing.
   
   
   
   
2. CyberDuck
   The duck is the second one on my list. Has similar features to Transmit but it sometimes does not allow connections to some Linux servers. Bad experience, plus the rubber ducky just reminds me of default msn avatar ><.
   
   
   
   
3. Fetch
   I use to use this and I don't get why people like it. I was always frustrated how navigating through it and having to open another finder to drag and drop. The other thing was there is no duel screen.
   
   
   
   

Windows

1. Winscp
   Does everything including ssh connections. This is for those that are hardcore web developers.
   
   
   
   
2. Filezilla
   I like this interface better than Winscp. Its clean and they upgrade it quite a bit. The community/support is strong and always growing.
   
   
   
   
3. SmartFTP
   Yet another FTP client that supports backups and great amounts of file transfers at a time. The look and feel with this one is more of a Office 2007 look which is extra eye candy.