OpenSea is probably the biggest thing, it's the largest NFT marketplace and it uses web3 extensively.
Starting to figure out Clarity.so, which is a project management tool kind of like Notion but with some web3 features. Haven't done too much with it yet, but an organization I'm working with is using it.
Metamask is a browser wallet that also serves as your login and identity for these sites.
If you want to learn some code and concepts in smart contracts in a really friendly way (while providing a lot of good information), you can go through the tutorials at https://cryptozombies.io/, which works like FreeCodeCamp.
Smart contract code has some similarities but also a few things you need to do differently because of the nature of the blockchain. Like random number generation you have to be careful with since your code is public, and people could potentially game it unless you do it a certain way, also function visibility is very important because it might allow other apps to call your contract and/or certain types might cost gas (i.e. tokens) when you don't need it to. Also a smart contract is immutable once it's published, so you need to be very careful you get it right, or you will have no choice but to point people to a different contract (and even still that other contract could still do some damage). Also the way string comparison is done is to make a hash of each of the strings and compare those to see if they're equal. Just a few examples of many.
So it actually feels like I'm learning something different, and not just "okay, what do I need to call/integrate to add authentication on this new platform" for the tenth time, or "Oh, the coordinates for drawing to the screen start from the top left instead of the bottom left for this tool, good to know."
I mean it's not as different as learning something like Prolog, but still fairly different.