My favorite one, in languages that do support it, is the discovery (it took me such a while) of the ||= operator, used to assign a value only if not already set.
For instance
myVar ||= myDefaultValue;
Which if the same as
myVar = myVar || myDefaultValue;
// OR
if (!myVar) {
myVar = myDefaultValue;
}(Obviously, this little syntaxic gem is only useful for any variable that isn't supposed to get any falsy value). I'd really like to get this operator in more language (like JS, as many readers here are JS fluent)