What is a good world time app for Mac OS?

I recommend iStat Menus. It integrates very nicely with the mac menu bar and the Date & Time widget is very customisable and can be made to look identical to the stock Apple menubar app. You have the option to add as many time zones as you like and they are all accessible by clicking the time in the menubar (see screenshot).

iStat Menus 2 used to be free if you can find that version kicking around (needs a kernel extension tweak to work with 10.8 ML); otherwise, version 3 is a (reasonably cheap) paid app.

It also comes with a wealth of other activity monitors that can be placed on the menu bar which integrate equally nicely: both visually and functionally.

I like Hour Lite. It's FREE, simple and has cute UI :)