What is a good graphing software ? The one that has the ability to accept an equation (linear, quadratic, etc.) from users and output a graph for that equation (software equivalnet of T1-84 graphing calculator).
You could also consider Desmos (web-based and free): https://www.desmos.com/calculator
This is old but it randomly appeared on the Top Questions list, and I can't help recommending Spacetime. While Octave/Sage/GeoGebra/Matplotlib-and-friends, etc. are quite a bit more powerful, spacetime feels a lot more like your Ti calculator: you can do basic things like plotting, fitting lines, evaluating eigenvalues, etc. very quickly and easily, without having to learn much syntax at all. It also has an iphone version which is a whole load cheaper than a Ti calculator, but probably quite a bit more powerful :)
Consider onlinefunctiongrapher.com it is web-based and does not require java or flash.