In know some apps offer separate widget apps so you can move the main app to the SD card. Is there any weather apps designed like this? I haven't been able to find one, so does anyone know if one exists?
Separate widget app may be available in case when wigdet covers only a small subset of the main application functionality. However, for the weather app and weather app widget the main difference is the screen representation, the processing routines are almost the same. I'm sorry to say, but it's very unlikely to find the weather application with the separate widget.
Moreover, with the Froyo/Eclair moving into the obsolescence quite steady and newer version of Android having "unlimited" app storage in the range of 8-16GB, it's very unlikely the new developers will ever be inclined to make that kind of widget app.