You're welcome

[Glen]

Are there degrees of formality among the various responses given to ¡Gracias! ? I've heard:
De nada
No hay de qué
No de qué
No hay que hablar
In Costa Rica it's Con mucho gusto
What are some others?
Does the response vary by country, region or level of intimacy?

[AngelicaDeAlquezar]

I think there must be a few regional differences; for example, in Mexico I've never heard "no hay que hablar". But I don't think any expression would be misunderstood. When I visited Buenos Aires, for example, I was disconcerted the first time I heard "No, por favor", which sounded harsh at first listening. A little later, the meaning was obvious for me and I learnt to identify the kindness behind it.

There can be some differences according to how close you are to someone, but I think this is mostly heard through the tone of voice. Usually, most expressions work in all contexts:

- De/por nada. -> Depending on the tone this may be felt cold, but it could be said in a warm tone, with the right gesture, and you won't need to say more.
- No, de qué. / No, de nada. / No, por nada. -> No me (lo) agradezca(s) (Don't thank me.)
- Ni lo mencione(s) -> Similar to the one above, but more polite.
- No hay de qué. -> This one usually sounds polite in any context.
- Faltaba más. -> Es lo menos / lo mínimo que puedo/podría hacer. (It's the least expected from me.) Colloquial, but polite and very friendly.
- A sus órdenes. / Para servirle. -> This is a common reply from people who has done a service to you, so it's usually heard for "usted", although it can be formulated for "tú".
- Fue/es un placer. -> My pleasure.

...and sometimes it's just a mild gesture or a smile what replies to "gracias", more than any spoken words.

[JPablo]

Algunas otras variantes:

-No, por favor... faltaría más.

-No las merece.

-Ha sido un verdadero placer.

-No me des las gracias a mí, yo no he hecho nada... (dale las gracias a Angélica que te lo ha explicado más a fondo...)

-Muchas gracias.
-Muchas "de-nadas".

-Muchas gracias.
-Las tuyas, salerosa... (si le contestas piropeando a una chica...)

And so on and so forth...

-¿Cómo que "gracias"? Son $50 del ala por los servicios prestados...

[Glen]

Wow, what a collection to add to my repertoire. Thanks a lot! I can't wait to try them all out.