This is a bit of an odd one. But if your family's roots are in England (and they may be, even if you don't know it), this is worth a look. Especially if you suspect some distant relation had a tendency towards thievery, burglary, larceny, or perhaps murder. If so, you just might find him or her in the Proceedings of the Old Bailey, London's Central Criminal Court (1674-1913).
There are about 200,000 criminal cases here, and well over a million names of defendants, victims, witnesses, judges, and attorneys mentioned throughout. William Shakespeare is here, on trial for pocketpicking (though not that William Shakespeare). It's so much fun to read these, that you should pay the Old Bailey a visit, even if your family is not likely to be mentioned here.
The Proceedings also include the collection of Ordinary Accounts (1679-1772) which can also be searched separately. These are fascinating journals of the last days of criminals condemned to death, and about to be executed. Reading them, it's often hard to tell who is the worse offender: the criminal or the court.
Another day at the Old Bailey.
Visit the main page of Free Genealogical Tools for more, umm, free genealogical tools.
And don't forget to also check for your family history at Ancestry.com and NewspaperArchive.com.
These are subscription databases, but they are among the most powerful research tools available for looking into family roots.