Fun to modify

Beyond design for modifiability.

The worst is closed-source software. Microsoft.

Next better is software for which source is provided, but you are not permitted to modify it and recompile it. Tivo. But at least you can see what the software is doing so you can decide whether to run it, or trust it, and understand and work around bugs.

Next better is open-source Free (as in speech) software.

Next better is Design for Modifiability, mentioned earlier. Well-documented clean modular code with lots of hooks is one possible characteristic. An easy mental model of how things work.

And now, a new category beyond that: Fun to Modify. Low barrier to entry to modify, instant gratification. This one is psychological.