What cryptocurrencies have no fees?

As a keen observer of the cryptocurrency market, I'm curious to understand which digital currencies operate without any fees. It seems counterintuitive in a world where transaction costs are often a defining factor in the adoption of various cryptocurrencies. So, could you elaborate on which specific cryptocurrencies boast a no-fee model? How do they achieve this, and what are the potential implications for their scalability, security, and overall usability? Furthermore, are there any caveats or limitations that investors should be aware of when considering these no-fee cryptocurrencies?