Here are a few i've used before:

Google charts: http://code.google.com/apis/chart/ (if you want a really nifty flash tool to help you create google charts check out: http://www.clabberhead.com/googlechartgenerator.html)

Sparklines: http://omnipotent.net/jquery.sparkline/ (on HN right now)

Flash charts: http://maani.us/

PHP Charts: http://naku.dohcrew.com/libchart/pages/introduction/