Crypto-currency interoperability My question is about Bitcoin, Litecoin and Ripple, to which other currencies (crypto-currency and regular) they can be exchanged? Are they exchangeable with any currency or can each one be exchanged with specific currencies?
There are services such as ShapeShift that have built whole businesses around exchanging cryptocurrencies for each other. 

There are numerous exchanges in many different countries that allow customers to trade national currencies for cryptocurrencies.

There are several payment processors that accept cryptocurrency payments for their customers and reimburse them in national currency.

There are some services that automatically convert your salary into cryptocurrency, allowing you to be indirectly paid in cryptocurrency by regular companies.

There are services that allow you to pay virtually any bill or order from any webshop through them as intermediaries, paying the with cryptocurrency.

By using a combination of the above, you can virtually trade any currency, your labor, any service, produce or physical object for pretty much any cryptocurrency.