Plot

Simple name, great looking 2D scientific plots. I haven't used Origin that much but I imagine this as a light weight and free alternative to that program. I've used it for plotting research data with great results but I don't demand much from a plotting program.

The UI takes a little bit of getting used to because it works in a little bit non-intuitive way, but it's worth the effort.

Plot: http://plot.micw.eu/Main/HomePage

Igor Pro is kick-awesome.

But it is by no means free. It's like maple, only it makes better plots.