In my more youthful days, I was completely addicted to Chuckie Egg. The original author ported it to the PC. His site has disappeared off the web at the moment, but there's a mirror here (~200k).

My current best: 269040

http://www.eyezmaze.com/grow/v3/grow.swf

Just spent far too long playing this game. Maxing out all levels gets you 20,000 points and a sensual feeling of satisfaction. Maybe.