Non linear regression calculators.

Since none of the calculators I've downloaded on the iPads at school do nonlinear regression, I had my college prep class do a web search to find a nonlinear regression calculator.  They helped each other get to s calculator and then just let them spend time exploring the calculator. The kids had a blast checking out how changing one thing could change it all. 
Due to to a change of plans, I had to work with one class on absolute inequalities.  I had the students practice problems on SAS Gloss.  Some used a plain background, some selected a lined or one with graph paper.  They enjoyed having different colors for the background and for the ink.  I enjoyed it because I could easily change a sign, put a smiley face or a frown, adjust this or that and the kids were able to make changes or corrections without a problem.  I like that they did not have to use tons of paper but could do it all on the iPad.