Litecoin
Litecoin purports to be an alternative crypto-currency. The only difference from Bitcoin is replacing the SHA256 proof-of-work with scrypt, making it more vulnerable to attack from rich entities (on the scale of big corporations and governments). Because of this, and the fact that it was created as a Bitcoin ripoff instead of a testnet experiment, some people consider it a scam. Also, since it does not have any logical reason/plan for long-term survival, it logically functions as a pyramid scheme, rewarding early adopters while those getting in at the end are left with nothing.