Upgrading from:

Graphics`ImplicitPlot`

ContourPlot in the built-in Mathematica kernel now accepts equations.

ContourPlot can plot implicit curves:

Version 5.2 << Graphics`ImplicitPlot`; ImplicitPlot[x^2 + y^2 == 1, {x, -1, 1}, {y, -1, 1}]

ContourStyle is used in place of PlotStyle:

Version 5.2 << Graphics`ImplicitPlot`; ImplicitPlot[{(x^2 + y^2)^2 == x^2 - y^2, (x^2 + y^2)^2 == 2 x y}, {x, -2, 2}, PlotStyle -> {GrayLevel[0], Dashing[{.03}]}]