What Day Is That? by Nicholas Cooke
What Day Is That? allows you to enter virtually any date and it will tell you what day it is. For example if you input the date February 28th 2010, it will tell you that it was a Sunday. The app works for past, present and future. You can also see how many days ago or away the date you are currently viewing is from today (i.e. 10 years, 4 months and 26 days away). This can really save you time and a lot of hassle.
Developer Website: http://www.nicholascooke.com/Day/day.html