I wanted to build my own version of this Investment Growth Calculator.
I am proud of myself because I built this using Test Driven Development (TDD). TDD is when the tests are made first, and then the code is written to make the tests pass. When adding a new feature, a failing test is created, and then code is added to pass the test. For more information about TDD checkout Test First and Ruby Koans.
Basically I collected data from the Investment Growth Calculator and then created tests based off of that data. I then made my code pass these tests.
In my version, you can enter your age, initial contribution, and how much interest you expect to earn. Click Calculate and see how much your money will be worth when you retire thanks to the power of compound interest.