PreCYdent is a new legal research search engine that seems to have much promise. You can read about it here.
It appears to "rank" cases much in the way Google ranks web sites. For example, typing in "qualified immunity" in PreCYdent doesn't just give you a random list of cases. It gives you the leading cases. Saucier v. Katz, for example, is the leading modern day case on qualified immunity. It's also the first result.
If you type in "good faith exception to the exclusionary rule," the second result is United States v. Leon, which is the case that gave birth to the good faith exception.
It's not perfect, of course. For example, the second result for "excessive force" is Hope v. Pelzer. Pelzer is an Eighth Amendment case concerning cruel and unusual punishment: It has nothing to do with excessive force.
If you type in "commerce clause," United States v. Lopez is the first result. Unfortunately, Lopez is an irrelevant Commerce Clause case, especially in light of Gonzales v. Raich.
Still, it seems promising. Why not head over there, type in some key words concerning key legal concepts, and see if the leading cases come up.