If you need to create charts of SharePoint list data within your site, there are some very sophisticated server-side .NET tools out there for doing this such as
Dundas Chart for SharePoint.
If you just need a simple bar chart, you can do quite a bit with XSLT in a data form web part. A good example of how XSLT can be used in this way is the "form-based survey results" web part displayed on the home page of
this site. This web part was part of the Business Performance Reporting SharePoint template in the Fantastic 40. Writing custom XSLT like this can be a bit of a pain though if you are creating more sophisticated charts.
So here is another option:
LiteChart. It uses a data form web part bound to a list containing the data that is being charted. The XSLT includes a reference to a flash charting engine which does all the heavy lifting to output the chart. Since this uses out of the box web parts and web services, there is no custom code to deploy to the server.