I use Equation Writer for the TI-89. It's not free unfortunately but it allows you to construct complex mathematical structures visually with the whole catalog of functions supported. For example, in this program you can insert "lim(" from the catalog and it will display all of the arguments in the limit expression (the function, the variable, and the target value) as a box, so you don't need to know the syntax of the lim() function as lim(x,y,z)-- instead you can see it visually. Just a fast little program that saves time if you need to type something complex.