One of the best I found is at:
http://www.27bslash6.com/warzoneTowerDefense.swf This link died.
http://www.madcowinteractive.com/warzone_original/ Use this one.
If you play "ground only" it lasts a little longer, but it's very difficult. Getting over 100,000 score is almost impossible and I have spent entirely too long trying to figure out strategies on how to go further.
Well I found one that I can play almost indefinitely even though the enemy tanks are nearly indestructible.
I started playing on 12/29/2010 at 2am and finally walked away on 1/4/2011 at 10pm
For much of that time the game was on pause while I slept and went to work.
As you can see my final score is 223,143 I eventually gave up because I just got tired of it.
The trick was being able to send the tanks that made it through back to the beginning to start over again. Also to be able to create such a traffic jam new tanks wouldn't be able to enter the game. Towards the end 1 tank would die only every 10 minutes or so.
You can click on each image to see it enlarged.
What is your best score? Please comment if you've taken a shot at it.