Apropos to today (April Fool's Day): an RSS feed that contains national holidays and other contextual information for the countries around the world that I read news from. While I'm likely to know that it's April 1st or Christmas, I'm less likely to remember (living in the UK) that it's Thanksgiving tomorrow; it'd be nice to get some advance warning rather than having to figure it out from context.
A customisable version of this would let you insert your own events: birthdays or appointments that you want to be reminded about.