The 1st and obvious answer is that half the world would be night and the
other half would be day without change.

However, what would be the less apparent effects? For example, how would
it affect the ecosystem, the climate, polar ice caps.

Lets assume that the Earth stops spinning in a manner such that it is
the time [in terms of sun/earth orientation] always 4:30 AM
standard-time in Stamford, Connecticut. Let's also assume that the
Earth's movement stops when it is summer for the southern hemisphere and
winter for the northern hemisphere.

What would be the effects? That it would be always be summer in
Australia and winter in Canada is self-explanatory.

Thanks,