Depends on what you're looking for. If you only want the display of the temperature of a certain city in your menubar, then the above-mentioned Weatherbug is more than enough. There are other, better weather apps like Weathervane, Weatherdock, Weathersnitch and my personal favorite, Meteorologist.
Personally, I'd take Meteorologist over any other weather menubar apps. Why? It is very customizable, offers city-cycling (you can have more than one city saved, and it will cycle all of them every min or more - depends how you set it), and is just neat. The only downside to it is that it sometimes gets stuck when updating the weather conditions, but usually quitting the app and relaunching it fixes this problem.