Building on top of my platforms post an example of one of the greatest platforms ever, the awesome, classic, long-lived LEGO. I really like the LEGO example because it works in the physical world where there are many more limitations than in software. Software can do anything, right? Right?
The Lego framework is simple: pieces attach to other pieces by snapping a stud into a properly sized space in another piece. It's a very limiting framework in that one can't just glue any two pieces together in any desired configuration. That's the beauty of it though, it takes a bit of creative engineering to make something artful and/or functional.
The extensibility of Legos is built into its framework. One can take any two pieces that interlock and connect them. The pieces aren't defined as "bridge" pieces or "house" pieces. A block is a block and a fence like piece doesn't necessarily have to form a fence. The flexibility is immense while still playing within the rules of the framework.
I'll define the infrastructure here as the pieces themselves. The precision design and manufacturing of those pieces is the hard part that Lego Group does for the customers. Once pieces have been purchased, one can do whatever is desired. The very clear instructions to build specific items may be considered part of the infrastructure as well.
Here's the beauty of Legos. Each box is itself a killer app. Lego Group makes their money off of selling these highly desirable killer apps. Kids and parents go to the store to buy a pirate ship or a Tie Fighter and then start using the pieces in that manner. After some time the new designs and home-thought configurations evolve on their own. The pre-designed sets provide proof that one can build cool stuff with the pieces and teach the lessons to extend from there.