A few...
Williams Blast Off in ratty condition from an NYC pin guy who needed space. It was savable and I had it playing within a few days. He offered me a choice of this or a similar condition Diamond Jack. I preferred the space theme. Fixed it up, played it a while, sold it on ebay for $600.
Strange Science - from a relative who was moving. About 10 years after I first asked for it...for no real reason they said yes and then no at the time (people tend to get stingy when they have something useless that they realize has value to someone else) but they came around eventually. the whole thing reminded me of a Laser Disc player that had sat in my aunt's closet for close to 10 years that she refused to give me. By the time she asked if I wanted it it was already the DVD era and I had no need for it. I assume it ended up in the trash.
This one worked great even the topper just needed a shop out. Played for a while and sold it.
Bobby Orr - this was really a gorgeous game and I believe it was HUO. Didn't work but I think a new board was all it really needed. Had been a gift from a Bally exec to this dude in the 70s. He was moving and found me and asked if I wanted it. played for a while and sold it
Night Moves cocktail - this one was in a flophouse of my sister's friends in college at U of Maryland. Nobody seemed to know how it got there or how long it had been there but they were just using it as a table. It booted but obviously needed work. technically wasn't free I think I brought them a bottle of Jim Beam. This game I had for about 5 minutes flipped it over to Todd at TNT (he says that's a good seller for him, go figure!)
F-14 - this one was sitting in a dumpster about 20 feet from my front door. That was a bit of a surprise. The whole game was there minus the speaker/display panel and translite. Decided against resurrecting it but to this day 7 or 8 years later I still get parts off of it. Every board worked too - most went into a Cyclone that had a bunch of iffy boards. I still have the playfield to this one and it's actually very nice (no wear or lifting inserts) could be super nice if someone took off the mylar. Maybe I'll offer it up here once it's fully stripped.
The best score is probably a toss-up between the Bobby Orr and the Strange Science. They were both really nice.
The key to accepting free machines is to be gracious, and accomodating and helpful when it comes to removing it- but never offer money, and never make any promises "Oh I'll never sell this never ever ever!!!!!" And make sure it's REALLY free you don't want to get into a situation like Seinfeld and that guy with the soup and sandwich at Mendy's.
Or just stumble across one in a dumpster.