Opensea is the most popular NFT marketplace on Ethereum. Just below Opensea are Rarible, Super Rare, Nifty Gateway, and Foundation.
It’s not uncommon for other blockchains to host their own NFT marketplaces. On Solana, there’s Magic Eden, Solsea, Solanart, and the upcoming Fractal platform, which will allow users to buy mere fractions of NFTs.