Free online graphing calculator:
The first one is easy to use.  It will graph any function and will find intersections.

http://my.hrw.com/math06_07/nsmedia/tools/Graph_Calculator/graphCalc.html

The second, is not as easy to use, but will find domain and range, x and y intercepts as well as even and odd symmetry, end behavior, asymptotes and derivatives.  It virtually analyzes the functions in every detail for algebra and calculus.  It is quite the free online tool.  The first link takes you to the main page where you choose the type of calculator you want.  The second is the direct link to the “basic” graphing calculator.

http://webgraphing.com/

http://webgraphing.com/graphing_basic.jsp

Free online scientific calculator:

This is an easy to use scientific calculator that has all the functions needed for all levels of algebra.

http://www.mathopenref.com/calculator.html