There's the flot plugin for jQuery, and Google's Chart API, but I've still never seen a Javascript or Web Service graphing library that does even the most basic of error bars.
FusionCharts (Flash) looks quite good at first inspection, if expensive.
It would be great to have a web-based database/spreadsheet application with GraphPad Prism's (excellent, but a bit tricky to use) statistics and graphing functions.