Chart2D
Chart2D is a .NET component written in C# that is used to draw and present line charts in your .NET Windows applications.  Chart2D is provided with complete source code so that you can customize and redistribute it to meet your own development needs – all for free.

DaveChart
DaveChart is a free .NET charting library.

NPlot
NPlot is a free charting library for .NET. It boasts an elegant and flexible API. The library (and it's source code) is released under a BSD style license. NPlot includes charting controls for Windows.Forms, ASP.NET and GTK#.

XSChart
XSCharting Component written in C# to provide a variety of charting options.

ZedGraph
ZedGraph is a set of classes, written in C#, for creating 2D line and bar graphs of arbitrary datasets. The classes provide a high degree of flexibility -- almost every aspect of the graph can be user-modified. At the same time, usage of the classes is kept simple by providing default values for all of the graph attributes. The classes include code for choosing appropriate scale ranges and step sizes based on the range of data values being plotted.