iPhone Moon Phase is an iPhone web widget, that will display the current phase of the moon. It will also allow you to enter any date, and see what the moon phase will be. There is also a countdown feature that will tell you how many days are left until the next full moon.

http://www.microseconds.com/imoon/

For all you beachgoers out there, TideApp looks up the tide info for your local beach, and is VERY comprehensive!

http://tideapp.com/

iWeathr.com provides quick access to animated doppler radar from weather.com from any location in the United States using your iPhone.

http://www.iweathr.com/iweathr/