Is there an iterative graphing program that lets you graph custom fractals like the Mandelbrot set? There are many really cool fractal escape time fractal programs, but I'm yet to find one where you can enter your own equation? 

fractint is one of the oldest programs, it has a formula interpreter.
xaos also supports user-settable formulae.
shadertoy probably has many escape-time fractal experiments on it that you can edit in your browser.
fragmentarium has 2D escape time examples, though its focus is on 3D distance estimated fractals.
These last two will require some simple programming to translate the formula into the programming language GLSL (OpenGL shader language), if you find the right example to start from that could be as simple as modifying one line of code.  They can support the things I mention below, as they are really programming environments and not just simple fractal formula interpreters.
Note that some formulas require special handling (eg, searching for periodic behaviour rather than "escape" to infinity, or starting from location-dependent critical points).  The (art-focused) programs I mentioned will probably display something for those cases, but it might not be mathematically meaningful.
