UPDATE: It looks like MMMOOO is running a sketchy promotion here where they require you to rate their application a 5 to enter a contest to win a free license. Sorry for the confusion but they clearly misrepresented the deal.
Putting the weather on your homescreen has become a hot fad and I really hope RIM finds a way to incorporate it natively in OS 6. Until then MMMOOO has us covered with Weather Plus and BBSites noticed that they are giving away free copies of the full version of the app from what I can tell. All you have to do is just email them after installing it.
Weather Plus Lite lets you put weather info on both your homescreen and on an icon and does not need a weather slot in the theme. They support multiple cities and it automatically updates the weather in the background.