Liquidity pools
One of the first projects that introduced liquidity pools was Bancor, but they became widely popularised by Uniswap.
Curve realised that the automated market making
The other idea for different liquidity pools came from Balancer that realised that we don’t have to limit ourselves to having only 2 assets in a pool and in fact Balancer allows for as many as 8 tokens in a single liquidity pool.