Graphing calculator (Desmos):

 https://www.desmos.com/calculator

 

 Another graphing calculator: (Houghton Mifflin Harcourt):

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

 
 For graphing, statistics, matrices:
  meta-calculator.com


Using the TI-83 or the TI-84 calculators: 
     1.  Online tutorial for using the TI-83/84 Calculator (from Prentice Hall)
     2.  Finding Your Way Around... (from Math Bits)
     3.  Downloadable pdf file of the TI 84+ Guidebook (from Texas Instruments)

Three online calculators for finding a line of best fit:

     1.  http://www.alcula.com/calculators/statistics/linear-regression/

     2.  http://www.mathportal.org/calculators/statistics-calculator/correlation-and-regression-calculator.php

     3.  https://www.easycalculation.com/statistics/regression.php