The Polkadot blockchain is a multi-chain blockchain that is designed to bundle multiple chains together so that they can be executed in parallel. The goal is for every block on the Polkadot blockchain to contain multiple blocks from other chains. This will enable the network to contain the transaction information from multiple chains and dramatically increase the overall interoperability between different blockchain networks. Polkadot was created by the Web3 Foundation, which is led by Gavin Wood, one of the founders of Ethereum. Polkadot is their flagship project, and aims to solve the issues in the blockchain ecosystem related to disjointed governance and interoperability.
Cosmos is a cross-chain protocol that is working to establish a standardized way for blockchains to inter-operate. It has been called the internet of blocchains. Cosmos is building a standardized protocol for multiple blockchains to operate freely together, and allow transfers of value between them. Cosmos is made up of a network of blocckhains called zones. Each of these zones will be powered by a the Tendermint consensus protocol. Cosmos will act as a hub, that ties the zones together and manages their coordination. The Cosomos token called ATOM was released in an ICO that raised $53 million, but it will not be traded until the platform is launched.