Until tomorrow (Jan. 5th, 2011), the Graphing Calculator Pro iPod app is free (thanks Josh). It can do a lot, so it might take a minute figure out how to do the graphs and it can be a little fiddly (you can scale both axes separately, for example, which is nice unless you don’t intend to), but I like it. After just a couple of minutes, the student who found it was already playing with slopes.