http://bcalc.net/   Note:  This cannot be used with Internet Explorer.

http://www.meta-calculator.com/online/  Note:  This will do matrices and statistics too!

 videomathteacher.com  Note:  This is a downloadable TI-83 calculator.  Click and then follow these instructions:
Click on the red box "Free Resources"
Then "Get Calculator"
Then "Virtual TI graphing calculator"
You will have to download the folder, then unzip the .exe file within the folder.