RnD is a clone of four separate Boulder Dash-inspired games (Boulder Dash itself, Emerald Mine, Supaplex, and Diamond Caves 2) whose mechanics can even be used together in the same level, with its own mechanics tacked on to boot. There's also a pseudocode system that allows you to make custom mechanics, e.g. diamonds that run away from you when you come near.
Can be found
here. There are also
tons and
tons of levels made for it, although their quality may vary.