It's just great. Directed by Frank Capra, it's got an all-star cast including Lionel Barrymore, Jimmy Stewart, and Jean Arthur. The supporting roles are filled with familiar faces, too -- you find Ann Miller, who dances quite a lot (though she's so good that she can't quite fool you into believing that her character is really a bad, bad dancer) and Eddie "Rochester" Anderson is a nice surprise - both he and Jimmy Stewart are lots younger here than how we tend to remember them now.
It's a witty film without being weighty. The whole family can watch this one and get something out of it. And, the play's even got an online study guide (learn such things as this movie being created as one of the origins of today's TV sitcoms).
I couldn't find it available for free on the web -- yet. But that's just a matter of time, right?