Underfell doesn't appear to belong to anyone. So feel free to make anything for it, or suggest something I should add to this in the comments! For now, though, you'll mave to make do with a few tumblr blogs.
They're trying to Underfell, and doing wonderfully.
Undertale has a VARIETY of things about it! Here's some great sites.
Official website
Bittybones is an adorable AU, and not just limited to Undertale related things! The official tumblr is below, but anyone can make one.
Bittyboooonnnnees
Underswap was originally created by popcornprince1 on Tumblr, but he abandoned it. However, Team Switched is making an Underswap game! It's not done yet, but you can check out the demo.
The Demo!
Horrortale is an AU set many years after a neutral ending of Undertale. It was created by Sour-Apple-Studios on DeviantArt.
The first page of the comic
The maker of this page has written some fanfictions and created some AUs. If you're interested, they are here.
All of these lead to the same author, so just click the author link to see the others!
Hyper, not technically an AU0