I just came across gauge.js:
Gauge.js 1.0 allows you to add gauges (with shading and reflection) to your webpages. It uses unobtrusive javascript to keep your code clean.
It works in all the major browsers - Mozilla Firefox 1.5+, Opera 9+, Safari and IE6+. On older browsers, it’ll degrade and your visitors won’t notice a thing.
These gauges might look familiar if you’re an iTunes user as they mimic those found when filling up your iPhone or iPod.