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.
THEKEY (TKY) is the token of the Chinese government endorsed blockchain based identity verification ecosystem. The token is currently based on cryptocurrency NEO protocol. It is listed on a few exchanges paired against major crypto coins.
TRON is a blockchain-based entertainment platform with its native token TRX. The mainnet was launched in 2018. The platform is supposed to be a cheap way to consume entertainment content and easily share it. TRON is capable of hosting entertainment content and DApps.