Beyond Compare is absolutely first class.

When modding html code I use it to update my live websites rather than the ftp side of my web design programs.

It saves me absolutely hours of work. syncing image directories and comparing code there is nothing better.

I like it seems a lot of people are saying - port it to OSX - it will be worth the investment - I know loads of people who would buy it.

I design on the Mac then have a pc to solely run this software - it is that important to me.