One of the things that I admire about Python is its distinction between mutable and immutable types. Having spent a while programming in c before coming to Python, I was astonished at how easily Python does away with all the complexities of pointer dereferencing that drive me mad in c. In Python everything just works the way I expect, and I quickly realized that the mutable/immutable distinction plays an important part in that.
There are still a few wrinkles, of course (mutable function argument defaults being a notable example) but overall, I feel that the mutable/immutable distinction greatly clarifies the question of what variables and their values are and how they ought to behave.
But where does it come from? I have to assume that GvR was not the first person to conceive of this distinction, and that Python was not the first language to use it. I'm interested in hearing about earlier languages that used this concept, as well as any early theoretical discussions of it.
Objective C is loaded with mutable/immutable distinctions (to the point where there are both NSString and NSMutableString, for example); it predates Python by about 8 years. Smalltalk, from which Objective C inherited much of its OO design, uses the concept to a lesser extent (notably, strings are not immutable; the trend these days is towards immutable strings as in Python, Ruby, etc.).