[DISCLAIMER] This is in no specific order, just things I've used/seen/heard about recently that I think are really cool. If I've missed anything it's because I have the memory of a goldfish or I haven't come across it in my feed yet, this post is subject to change with new information!
Blacksky
Not only is it amazing that we're seeing exactly how communities on AT Protocol are playing out but we're also seeing a lot more people learning about the AT Protocol which is awesome. I've seen people sharing all of the tools for PDS migrations and helping each other move to the Blacksky PDS which has been very cool to see. Also at the time of writing this they JUST released their account migration tool https://github.com/blacksky-algorithms/tektite-cc-migration-service ! This makes a total of 4 account migration tools that exist today. There's never been a better time to migrate your account if you've been wanting to move away from bluesky hosting your data.
NorthSky
Similar to the goals of Blacksky, NorthSky are also spinning up their own infrastructure separate from bluesky to provide a safe social media experience for 2SLGBTQIA+ people. They also have a pds migration tool to help users migrate from bluesky to their pds which is in alpha https://github.com/NorthskySocial/pds-migration.
Tangled
The speed at which things are being developed over at tangled is insane. I said I'd move over fully from Github once CI landed and it was generally available a few days later. Any new projects I start will now primarily be on tangled. The people in the discord are super cool as well so consider joining! https://chat.tangled.sh/
Grain
I feel like Grain doesn't get enough love. If you're someone that's into photography or just like taking cool photos every now and then, grain is where you want to be. It's being actively developed and now has an iOS app that is in beta. Check it out at https://grain.social/
Graze
I'm pretty sure this is the de facto way to create feeds for bluesky for people that want an easy out of the box experience. Graze has been around for a while now and continue to push the bar for feeds. Bluesky let's you curate what you see on the app and with Graze you can build exactly what you want to see. I haven't had the need to use it myself because a lot of the feeds I want to use exist already but if I were to make one I'd likely use Graze.
Smokesignal
Smokesignal was probably the first AT Proto app I learned about when I joined bluesky. I'm not even sure how I found out about it, I think Nick was posting about it on bluesky and it ended up on my feed so I decided to check it out. Not only is smokesignal amazing for meetup style events but Nick writes about the AT Protocol on the smokesignals blog that is 100% worth a read. Nick is also the creator of AIP which makes it super easy to add OAuth to your atproto apps.
BookHive
If you're super into books then this is where you want to be. I think BookHive is meant to be a goodreads alternative but on AT Proto. I don't read as much as I like to but for book tracking this is the website I'd definitely use.
Streamplace
Honestly, how live streaming is even possible on at protocol is magic to me but still super exciting to see. The streaming experience as a viewer is super smooth and from time to time I'll pop into some streams to lurk and see what people are livestreaming. You can catch Eli (creator of streamplace) live streaming working on streamplace or playing video games quite often!
Spark
I'm not a massive fan of tiktok style feeds but I'm interested to see what spark delivers. They've talked about giving users more tools to control what they see which is great! Also tools around controlling how much time you spend on the app. Updates on where they're currently at are a bit sparse but this app has a ton of potential behind it. They've got a smart team of developers and I'm hopeful that the launch will be awesome.