One of those "why didn't I think of that?" things is UmbrellaToday.com. You hit the site on your mobile phone, enter your zip code, and it tells you YES or NO to one question: will I need an umbrella today?
You can also enter your cell phone number to receive daily weather reports at a certain time. (Say when you are getting dressed in the morning.) Since in many cities the weather is pretty much a mystery each day and you can end up dressing in a sweater in the morning and wishing you were in shorts in the afternoon, this could be handy.