We have a Supreme Court confirmation coming up, so we're hearing even more than usual lately about "activist judges." This is the construct used by the Right (and repeated, ad nauseam, by wingnuts who have little to no idea of how the judicial system actually works) to negatively label any judge who rules contrary to their worldview (which, incidentally, has very little to do with the US Constitution, but I digress). Truly, courts are "activist" all the time, on both sides of the political divide, if you want to look at it that way. It would be better to understand how the judicial branch works, to understand that some of the most significant cases in history, cases we take for granted, for better or for worse, are all examples of "judicial activism."
Properly understood, “judicial activism” is neither liberal nor conservative, nor is it always bad, or at odds with the ideals of the American Constitution. Granted, sometimes it is.
Marbury v. Madison, 5 U.S. (1 Cranch) 137 (1803): could Marbury, the defining case of American constitutional law, be anything but #1? In Marbury, the Court claimed for itself the power to both interpret the Constitution, and invalidate those acts of Congress at odds with the document. It invented “judicial review,” a power envisioned for the Court by the Founders, but not explicitly granted in the Constitution. You don’t get more “activist” – or more vital to our liberty – than that.