It's a huge space and I got into it around Feb/March of 2021. Some interesting applications I noticed are:

1. Lending Apps - A trusted decentralized application to deposit collateral and earn more interest than a traditional bank. And then use the same application to take a loan on the basis of the deposited collateral. Example Apps: Compound / AAVE / TraderJoe.xyz to name a few

2. Exchange Tokens - A simple application to swap between two tokens (ERC-20). Example: Uniswap, QuickSwap, etc

3. Ownership of digital goods - Imagine if all the digital goods someone owned in Farmville could be sold, traded, exchanged. All of this is possible due to NFT's - ERC 721 Standard Example: Aavegotchi, Axie Infinity, etc

Interesting research is happening in the field of cryptography - Zero Knowledge Proofs, Verkle Trees. Foundations/Companies: Matterlabs, Arbitrum, Optimism, Polygon

A lot of work is happening in app development tooling: Solidity, Ganache, Brownie, etc