Look here.

http://www.sagemath.org/

I just looked a little.

It looks good.

It's free.

You can make worksheets online.

Click where it says, "Try Sage Online".

It will ask you for a user name and a password.

Then, immediately you can make a new worksheet.

Once you get the worksheet, trying typing, "2^2^2^2^2", or, "factorial(8000)", in the box.

Then, click on, "evaluate".

I think you can save worksheets online too.