Languages that challenge you to think differently. According to AlanPerlis (EpigramsInProgramming) these are languages worth knowing.  This is ''not'' the place to debate the merits of the languages from a practical standpoint, but to merely point out those that may challenge your world view and expand your thinking (software-wise, or perhaps deeper?). Perhaps it helps you solve problems better in "regular" languages, or is just for the joy of MentalMasturbation.

* LispLanguage (and its descendents, notably SchemeLanguage)
* PrologLanguage
* AplLanguage (and its descendents)
* SmallTalk
* ForthLanguage
* ErlangLanguage
* HaskellLanguage
* StructuredQueryLanguage (plus the RelationalModel and DataManipulationLanguage''''''s in general)

Some 'toy' languages that qualify.
* ToonTalk
* LogoLanguage

Of course this is relative. Most are trained on AlgolLanguage descendents, such as the C family, Pascal, VB, etc. If one was originally trained on say Prolog, then the Algol-style may seem mind-altering when first encountered.

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So, a MindAlteringLanguage has to be slightly esoteric, cause he won't follow the Algol pattern. Maybe there is space for a LittleMindAlteringLanguages that are still Algol like, but give you some new idea. 
I wouldn't have thought of using metaclasses before I knew python (even if they already existed in CLOS)and I wouldn't have thought of building my own control structures before I knew ruby's blocks (even if they already existed in ST) 

Also, I think in the mind altering space you can't omit to name ToolCommandLanguage.
The fact that it is homoiconic, that every control structure is a command, that there are no keywords at all, and that it uses "set" instead of "=" or ":=" is surely something that changes the way you think :)

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I think that the most mind altering of the languages mentioned, is Forth. You have to manage your own stack, for God's sake!!! It makes assembler look a little better.

''It's all relative.  I think the most mind altering of those listed above is Prolog.''
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I wish I could ask Timothy Leary ;-)