Amendment IIIX or VII if you like.

In suits at common law, where the value in controversy shall exceed twenty dollars, the right of trial by jury shall be preserved, and no fact tried by a jury, shall be otherwise reexamined in any court of the United States, than according to the rules of the common law.

What it means:  Pretty straight forward…any lawsuit involving the sum of more then $20 has the option of a trial by jury, and none can take this from you.  Additionally, this can not be changed, and any re-examination of the case of the facts thereof are to be done by a set of rules. 

Today:  Ooh, much abused in todays lawsuit crazy country, for two simple reasons.  Juries tend to be easily swayed by rhetoric, and tend to award outrageous claims.  Not to mention a good lawyer can pick a friendly jury, with their friendly little biases.  I think it is a paraphrasing of Mark Twain that in order to have a trial by jury, first you have to find the most ignorant people around…

Of course, since lawsuits are so costly just in the filing, most people just settle out of court as more cost efficient, EVEN IF THEY WOULD WIN.