Obviously there are all kind of cute games that can be played with pointers if you know arch-specific details (IIRC v8 was using alignment guarantees to steal low bits at one point, might still be). And tagged pointers of various kinds go at least back to Symbolics if not further.

One that I thought was especially nifty was [0].

[0] https://github.com/facebook/folly/blob/master/folly/PackedSy...