I just used flot a few days ago and was really impressed. Especially with how it does time series so easily. You just hand it a bunch of dates, which may include irregular intervals or gaps, and it figures out a reasonable representation for the data so it looks consistent. Really really nice and simple to use too.
There's a weird label positioning issue (label text stays a fixed width from the left edge of the screen, not the left edge of the chart) with Google Chrome, but not with Windows Safari or Opera.
I have yet to see free, client-side charting/graphing that works as well as commercial alternatives. We recently started using Flash-based charts called FusionCharts. They're a little expensive, but are well worth it if graphing is a central part of your web app.
http://developer.yahoo.com/yui/charts/
http://solutoire.com/plotr/
http://www.liquidx.net/plotkit/
http://code.google.com/p/flot/
http://www.ejschart.com/