This site offers a javascript library that allows you to draw vector graphics using nothing but javascript. It allows you to create lines, rectangles, ellipses, polygons, and arcs with options for filling or not (as appropriate). You can also include text and images in the resulting graphics.

This is a really powerful tool is yo are looking for a simple way to create vector graphics on a web page for things like bar charts, pie charts, or other interactive graphics without requiring plug-ins from the users.

If you go to the site, they have an interactive widget that allows you to draw things in real-time.