I realise that I lack some intuition into how a curve (or surface, or whatever) looks geometrically, from just looking at the equation. Thus, I sometimes resort to some computer program (such as Mathematica) to draw me a picture. The problem is, all these programs require input of the form $y=f(x)$, whereas my curve might be something like $y^3+x^3-6x^2 y=0$, and transforming this into the former form is not always easy, and always misses some information. So, my question:
Are there any programs that can take an equation ($p(x,y)=0$, say) as input and return a graph of its zero-set?
Update: So, lots of good answers, I wish I could accept them all. I'll accept Jack Huizenga's answer, for the reason of personal bias that I already have Mathematica available.