Polkadot is a network of multiple blockchains which can easily communicate with each other.
Kusama is Polkadot's canary network. We participate in the 1000 Validators Programme, which helps new validators to get in the active set and gain a reputation.
This is the latest meme fork of Nano. Nice community but still very small.
Banano is a meme fork of the Nano cryptocurrency. It has a wholesome and large community, despite not being a hyped meme coin.