I admit, this one is a little bit macabre. But it's something I was thinking about, and I was thinking about programming at the same time, so I decided to write a C# app for it!
You can use the Setup function of the app to set your birthday in UTC, down to the second if you know it, and how many years you anticipate spending on this mortal coil. (A figure which is subject to change, obviously.) The settings are saved in HKCU so you do not need to reset the info every time you reopen the app. Warning: It's a little creepy when you realize the percentage has crept up every time you hover over the hourglass.
That's pretty much all there is to it. 64-bit Windows and .NET 4 are required. There's no good reason for that other than that I don't like 32-bit Windows and < .NET 4. It's 2012, people.
As always, feel free to hit me with bug reports and enhancement requests.
Here's the executable: Mortal Countdown.exe (40.50 kb)
And here's the source (VS2010): Mortal Countdown.zip (64.30 kb)