This thread gives a link to Fantastic Contraptions, and what a fun game it is! After a few levels, I started trying to use it as a sandbox of sorts, messing with the pieces to build interesting things out of the game context, just for my own fun. I had more fun messing around like that than with the actual levels!
That is why people buy things like the Mindstorm sets or mess around with the Robosapiens toys. I’d do the same, but I have little space and money.
So I thought: can it be that somebody made a program to do just that? A construction set for PC? I think I remember a LEGO program many, many years ago, but if I remember correctly it was very limited, and I’d like something more, how can I put it, mechanics-oriented. It wouldn’t even have to be very realistic; Fantastic Contraptions, for example, simplifies a lot of things (water rods? magical engines?), so it’s good, but it doesn’t give me much freedom (you can design your own levels, yes, but that’s not the same; you create a problem to solve, not mess with a contraption) and it’s in 2D, within a small window.
They’re all very good, but they all more or less are about solving puzzles. Even when you don’t play levels, you design your own puzzles. I was hoping for something more free form. Suppose I want to build a double differential? A little walker? A trebuchet?
The Incredible Machine does have a mode in which you just build, you don’t have to solve puzzles or design your own. At the bottom of the Wiki page is a link to this one for Widget Workshop, which “focuses more on the freeform mode than [The Incredible Machine].”
There’s a 2D physics simulator called Phun that sounds like exactly what you’re looking for. Search youtube for “phun” and you’ll see videos of the software in action. One of the coolest ones I’ve seen is this fully functional mechanical clock: http://www.youtube.com/watch?v=L70aUEPRQyU
Sounds like what you want isn’t a game, it’s an engineering CAD program. Those exist, but you’d need to specify closer to what you might want to design because none that I’ve seen have every different type of material and ways of putting them together.