Here is a list of three cool apps that I use all the time (they all have free and paid versions, the paid versions are all worth it)
Geometry Pad REALLY GOOD for creating geometric figures, can also be used for functions. This is the app I use the most and I suggest that if you can only get one of these three to get this one, but it depends on what you need an app for.
Scientific Graphing Calculator This is great for graphing, it also has a reference for learning from. It graphs in all 2D planes. It graphs isometric, parametric, polar, Cartesian, and inequalities. It also has useful tools such as the triangle solver, graph solvers, and matrix operations.
Quick Graph This is hands down the best graphing app in existence. It is better than a $100 graphing calculator and can even graph in 3D.
Here are some pictures I created, most of which aren't that hard to make even for beginners)
Scientific Graphing Calculator