Law: Legal Citator (LexisNexis)

You can:

Determine the validity of a case (has it been reversed on appeal?  been overruled?  been treated negatively by a subsequent court?)

Discover the history of a case (prior and subsequent)

Find parallel citations

Locate other cases that have cited your case (and are likely to deal with a similar issue)

Locate secondary sources that have cited your case & address your topic