What folding bike would you recommend? I have toyed with the idea for a while, but all bikes I have seen so far seemed to heavy in the end.
That's what I use. It does not fold and is slower than most normal bicycles. But very light weight and it's mechanics are easy to understand and repair even for me.
Bike Friday (http://www.bikefriday.com/) and Dahon (http://www.dahon.com/) are popular in the U.S.
In case you don't think folders are "real" bikes, note that they are ridden by many serious cyclists (http://www.bikefriday.com/node/4813).
I have a fixed-gear mountain bike with 700c x 42mm ("really big") tires, and that doesn't really affect speed much either. The wheels very clearly have a sort of flywheel effect, though: once I'm moving, they hold a lot of momentum. (It's excellent for commuting in snow, BTW.)
Still, they're nice: there's really not much to break (or spend time maintaining). They are also incredibly easy to keep stable on ice and snow, provided you have reasonable tires. If the wheels start to lose traction, the pedals lurch, and you reflexively stabilize them.