ChainLink (LINK) is the native currency of the ChainLink platform. It represents an ERC-20 token is listed on multiple exchanges paired against major cryptos. At the time of writing, you can trade LINK against the fiat USD currency on Coinbase Pro.
Smart contracts are not able to access data on their own. Since connecting these contracts to data via node is deemed as a point of failure, using a decentralized service could be a solution.
Therefore, the ChainLink project sees its mission in the integration of the off-chain data into smart contracts through a reliable decentralized oracle network. LINK tokens hit the market in 2017 and the currency quickly made its way to the top 50 cryptocurrencies by market cap. Later it reached the top 20. ChainLink partners with a large banking network SWIFT.
Tether is a cryptocurrency launched in 2014. It is a so-called stablecoin currently leading by market capitalization. Although according to the initial idea, each Tether was backed by $1 (that was going to keep the price of Tether stable), the company not always managed to maintain this price (at some point in October 2018 the price dropped to $0.9 per one Tether). Tether succeeds in terms of market capitalization and its price is almost all the time is really close to $1.
POA Network (POA) is the native token of an open-source POA network for smart contracts. The token is built on the POA project’s blockchain and listed on several exchanges.