It turns out supply-side theory is correct.

According to the Congessional Budget Office, tax cuts of capital gains taxes led to an increase in tax revenue over what was projected, says Investor's Business Daily.
IMPORTANT NOTE: The years mentioned are off by a year. The taxes were cut in 2003, but didn't take full effect until 2004, and the numbers should be for the years 2004-2005. Everything is otherwise correct.
Here's a nice little chart showing the increase:

That's an increase of over 20%!
The tax cuts not only helped the economy but more than paid for themselves!

Crazed Rabbit