Evolution of Rules

The Jeopardy! we all know and love has evolved a lot over the years. The show began in 1964 in a format almost exactly like the current format, with the following differences:

It was hosted by Art Fleming.
Values in the Jeopardy! round ranged from $10 to $50.
Values in the Double Jeopardy! round ranged from $20 to $100.
Contestants could ring in at any time after the clue was revealed, even before the host finished reading the clue. There was no timeout attached to the ring-in, so that player could wait to response until the reading was finished.
All three players took home on the money that they won, not just the champion. The winning contestant still returned on the show.

In 1972, a special 2000th anniversary show was aired featuring three former big money winners, this time playing for charity. For this show only,

Only two rounds were played, Double Jeopardy! and Final Jeopardy!
A Daily Double was hidden in all six categories.

This version of Jeopardy! was cancelled in 1975. In 1978, a short-lived follow-up version aired, also hosted by Fleming. Differences in this version are:

There were only five categories per round.
Clues in the first round ranged from $25 to $125.
Only the two highest scorers from round one advanced to round two. In this round, the clues ranged from $50 to $250.
The player with the highest score at the end of Double Jeopardy! was the winner, and played Super Jeopardy! (yes, not Final Jeopardy!) alone as a bonus round. A new board of five categories was presented, and the objective was to get five answers in a row (vertically or horizontally) before missing three. I don't remember what the prize was, though.

In 1983, two pilots for the new incarnation of Jeopardy! were taped, with new host Alex Trebek. (Art Fleming was not asked to host.) The show reverted to the traditional format again, with the following differences in the pilots:

Pilot one continued to use printed cards for the clues; the video monitors were introduced in pilot two.
In pilot one, the clues still ranged from $25 to $125 in round one, and $50 to $250 in round two. In pilot two, the values doubled to $50-$250 in round one, and $100-$500 in round two.
Since the start, only the champion gets to keep the cash; the others receive consolation prizes.
The pen and paper for Final Jeopardy! wagers/responses are replaced by electronic pens and slates.

In the fall of 1984, Jeopardy! hit the air again, where it has remained ever since. One more change was made at its debut:

The dollar values were doubled again, so that first round clues range from $100 to $500, and second round clues range from $200 to $1000.

Several other refinements were made during the first season.

Contestants cannot ring in until Alex finishes reading the clue.
Shortly thereafter, a lockout device is installed on the buzzers. It was possible to hold down the buzzer while the lockout was active, then when the lockout was released, that player was almost certain to get in. The lockout causes the signalling button to reset for 2/10 of a second if it is pressed too soon. This defeats that particular cheat, and explains why sometimes you'll see a contestant jiggle the button and not get in. You have to wait for more than 2/10 of a second before pressing the button again.
A lockout is installed on the electronic pens to prevent them from working after the 30 seconds has expired during Final Jeopardy!

The buzzer situation remained the same up until 1991, when something very interesting happened. They removed the 2/10 second lockouts from the buzzers, meaning that a contestant could ring in as much as possible before the buzzers were enabled, without penalty. This effectively returned the state of things to the way they were back in 1984. This change was obviously deliberate because the contestant coordinators explained it in great detail to the players! Of course, the really aggressive buzzer pushers must have loved it this way. They eventually returned to the lockout scheme. I suspect that the lockouts for the new podiums used on the 1991 set just didn't have the lockouts implemented right away.

Since then, the game has remained pretty stable. There have been changes during special events, such as the Super Jeopardy! tournament, but regular play hasn't changed much at all in over a decade. The newest change I can think of was the "Bonus Category" format introduced (and quickly dropped) in early 1998, in which there were two correct responses to each clue, and a player could answer both for double the money.