Links

Videos

Solving linear equations using graphing (time: 3:57) 
http://www.youtube.com/watch?v=IN3JpyOSnRw

Solving linear equations using substitution  (time: 8:11)
http://www.youtube.com/watch?v=cwHR_B9zK7k

Solving linear equations using addition (time: 9:59)
http://www.youtube.com/watch?v=ej25myhYcSg

History

This is one of the best all around sites that I have found. It is titled "The MacTutor History of Mathematics Archive."
http://www-history.mcs.st-and.ac.uk/~history/

This is an award-winning site that links to all sorts of mathematics information.
http://archives.math.utk.edu/

Euler:
http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Euler.html

Reference Topics

This site gives a nice review of solving simple systems of equations.
http://www.purplemath.com/modules/systlin1.htm

This site gives a review of solving simple systems of equations by adding with links to solving simple systems using other methods. This is a great site if you take this link along with the other links provided at this site.
http://www.analyzemath.com/Tutorial-System-Equations/Tutorial-System-Equations.html

Here is a site for solving systems of equations by graphing.
You need to solve the first equation for y and enter the result on the line called f1(x), and then solve the second equation for y
and enter the result on the line called f2(x). Finally, look for the point of intersection. http://cs.jsu.edu/mcis/faculty/leathrum/Mathlets/grapher.html