Liar Liar (1997) was about a lawyer who becomes physically unable to lie (or even conceal the truth) and the effects it has on his personal and professional life.

Honest Lawyer: Your honor, I object!
Judge: Why?
Honest Lawyer: Because it's devastating to my case!
Judge: Overruled.
Honest Lawyer: Good call!

There's the TV series Angel, in which the lawyers of Wolfram &Hart provided legal cover for creatures of the night. Believe me, you don't want to anger their senior partners.

One suggestion - that movie with a slightly over-the-top performance by Al Pacino, "Devil's Advocate", wherein Pacino plays Satan whom is disguised as New York based corporate lawyer named John Milton. Milton, who is obviously aware of the problems of medical malpractice lawsuits, proclaims that lawyers are his Army of Darkness.

"We're coming ouuuuttt!"

Another suggestions - NBC`s Law and Order. Though this is simply a show about Police and District Attorney`s prosecuting criminals, the show does contain a `fantastic twist`, that being, on the show sometimes justice is actually done.

The movie Daredevil, and the comic book series of the same name, describes the life of a not very successful blind plaintiffs/criminal defense lawyer whose accute sense of hearing allows him to know when people are telling the truth or not. When the jury makes a mistake, he uses his "sonar" and special tools to secure vigilante justice.

Debt of Honor, by Tom Clancy, involves as a central story line, a product liability suit aimed at a Japanese manufacturer that explodes into an international political incident; the sequel, Executive Orders, has an interesting legal subplot pertaining to succession to the Presidency in a situation where most of the Congress and cabinet are killed in a terrorist attack.

Magic, Inc. (see "Waldo and Magic, Inc., Robert A. Heinlein) is good, involving an attempt by the devil to influence the state legislature to change the laws regulating magic.

Also, you might try the Lord Darcy stories, by Randall Garrett. They're detective stories, but address questions of proof in a magical universe.

There is of course always Wolff &Byrd, Counselors of the Macabre. Very funny. Might be a bit too far for magical legalism, though.

The Larry Niven story Jonathan Dresner is thinking of is probably "Convergent Series."

I'm surprised no one has mentioned Stephen Vincent Benet's "The Devil and Daniel Webster."

James Blish's novel Black Easter.

Little known fact -- Law and Order is set in the same universe as The X-Files. It's true! Richard Belzer played the character of John Munch in both series, along with Homicide: Life on the Street. And since Mulder and Scully appeared as characters in The Simpsons....

How about Frankenstein, by Mary Shelley, where Frankenstein allows the family maid to be tried and executed for his creation's crime?

Charles Harness has written a number of law-based sf and fantasy stories. A couple I remember from 1980s Analog magazine was The Venetian Court and H-Tec.

Frank Herbert wrote The Dosadi Experiment, which ended in an unusual trial.

Cordwainer Smith [Paul Linebarger], in his Instrumentality of Mankind stories, wrote a few set around trials. See Drunkboat, though that seems more like canon law then secular law.

Certainly Through the Looking-Glass by Lewis Carroll [Charles Dodgson] is worthy of mention. It's even been referred to in legal briefs.
See the book Judgment at Berlin, about a US judge summoned to Berlin to preside over a hijacking case in the early 80s for an example.

In the thirteenth-century Icelandic Njal's Saga, Njal was

[S]o well versed in the law that he had no equal, and he was wise and prophetic, sound of advice and well-intentioned, and whatever course of action he counselled turned out well.

It's a great read. Everytime someone gets whacked, there's lawsuit for wrongful death.

I reccommend the "Penguin Classics" version translated by Robert Cook.

It's 90% pure fantasy, but Poul Anderson's Operation Chaos has a small amount of legal procedure and police action in a magical USA.

Each of the Star Trek series had courtroom drama episodes. Possibly the best was "Measure of a Man" from The Next Generation, where the android Data's status as a person was determined.

One of H. Beam Piper's Little Fuzzy books has a trial to determine an alien species' sapience.

There's an episode of the original Outter Limits called "I, Robot" (no relation to the Asimov story or Will Smith movie of the same name) where a robot is on trial for murder.

Come to think of it, one of the main characters in Dr. Jekyll and Mr. Hyde is Utterson, Jekyll's lawyer. And Jekyll's will -- holographic, because Utterson refuses to draft it -- provides a minor plot point.

There was another Asimov story about a bank robber, a Mr. Stein, who zapped himself 7 years and one day into the future to bypass the statute of limitations. The prosecution argued that since he hadn't aged 7 years, the limitations didn't apply. The defense argued that seven years had elapsed since the crime, so the defendant was no long prosecutable.

The judge ruled (please forgive Issac for this one...)"A niche in time saves Stein."

Someone already mentioned Daredevil, but there are a few other comic book characters who practice law when they're not out battling the forces of evil. Marvel's She-Hulk (a cousin of Bruce Banner who gained Hulk powers through a blood transfusion) works at a law firm that specializes in "superhuman law". In the pages of her most excellent series she's helped Spider-Man sue the Daily Bugle for libel, persuaded a judge to let a ghost testify at his own murder trial and defended a superhero (the demigod Hercules, actually) in a civil suit filed by a supervillain for use of excessive force.

Another currently ongoing comic series set in the DC Universe called "Manhunter" stars a frustrated DA who hunts down and kills the supervillains who routinely manage to escape justice by way of manipulating the law.

Another great comic series that dealt with "fantasy law" is "Top 10" by the legendary Alan Moore. It was a cop drama set in a town populated entirely by superfolk called Neopolis. Defense lawyers from the law firm of Fischmann (a humanoid shark), Goebbels (I'm guessing some kind of Nazi) and Metavac (a brain in a jar) all make frequent appearances.

Hope that helps.