Williams used the wireform on a couple pre-flipper games including Cyclone. The most memorable and desirable flipper game with a wireform habitrail is 1953s Williams Skyway.

http://www.ipdb.org/machine.cgi?id=2206

The list shows Bally as introducing the pop bumper, but I can't find any reference to what game it was. To the best of my knowledge, Williams Saratoga was the first.
http://www.ipdb.org/showpic.pl?id=2055&picno=7433

San Fransisco was the last.

http://www.ipdb.org/machine.cgi?id=2049

In the late 40s the knocker was not used to indicate replays. It was used as a "drumroll" sound effect on games such as Ali Baba and Buccanneer along with a few others. Max out and collect the bonus on Buccanneer and the knocker rattles of a ridiculous amount of times.

Now we're really getting into the unobtainable. lol. I looked for a long time for one. I think there was one at that big auction a few weeks back.

Another game from that era of Williams, Big Ben has a bagatelle at the bottom of the playfield.

Not mentioned yet are the disappearing pop bumpers on Gusher and Sea Wolf along with a couple others.

I'm still quite found of the swinging target on Magic Clock, the first game to have one, although they became pretty common after that. And drop targets that progressively increase in value up to a special on several 60s games.

A gobble hole behind a drop target on Jumpin' Jacks. I believe they were simply referred to as "gobble holes".

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1956 Fun House is an EM with a one of a kind feature not used on another game until the sequel Funhouse in 1990.

A trap door, or in this case three of them.

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Or even a game where 9 dancers lift up their skirts.
http://www.ipdb.org/machine.cgi?id=3815

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About the most unique thing I ever found on a Gottlieb were these little guys. Only used on two games.

http://www.ipdb.org/showpic.pl?id=1607&picno=28228

And the rock em sock em robots on Knock Out.

You just weren't paying attention then.

http://www.ipdb.org/machine.cgi?id=3815

First game with an on playfield reel was Fiesta and there were several more after that in different incarnations.

Star feature was only used on four games and gave you an opportunity to win up to 200 free games for one extra coin.

http://www.ipdb.org/glossary.php#Star_Feature

Every game I've owned on that list, I sold too.

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1957 Williams Arrow Head is not the only, but I believe it to be the first game that has a kicker between and below the flippers.

Yeah that's the Arrow I got from Don. It's right up there as one of my top woodrail players. Yes the kicker is below the flippers is what I meant.

Lady Robin Hood is the one with the kickout hole between the flippers.

Not mentioned yet is 21 that plays like blackjack against the dealer and your card score is kept on two score reels, while your point score is traditional light scoring. There were a couple more similar to that.

This is one that is very unique and was probably only used on one game.

Williams Nine Sisters has a "whirlygig" where a lower kicker propels the ball through it and a second kicker under the whirlygig propels the ball to the top of the playfield.

http://www.ipdb.org/machine.cgi?id=1679

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That timer was very common feature on older push slide coin operated games. I think the point of it was to shut the game off when it wasn't in use. Sucks when you start an excellent game and get a little sidetracked and then it shuts off before you are done.

How about a pinball machine with an actual mechanical running man unit?

http://www.ipdb.org/machine.cgi?id=2209

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