Icons8 is an icon and design resource company whose blog covers design tools, icon sets, and product lessons.
HN commenters recommend the Icons8 site as a good source of icons, and note its freemium model and a DMCA mishap.
For those looking for a linux-compatible solution, I've had some success with Avalonia: https://avaloniaui.net/
Specifically I made a small application with F# and the FuncUI library https://github.com/fsprojects/Avalonia.FuncUI
So far the most complex real-world application I've seen is Lunacy: https://icons8.com/lunacy
It's nice to have something like this out there. It definitely eased my apprehension of using the platform on the grounds of it being immature.
I've a decent amount of experience with Xamarin (Native and Forms), and so far I've had 1/100th of the build issues with Avalonia.
I am excited to try out MAUI however and see how things have evolved!
Quant-UX is worth a look for prototyping needs, it's OSS as well and can generate end HTML/CSS code: https://quant-ux.com/
Lunacy supports multi-player in the way Figma users are used to: https://icons8.com/lunacy
Frame / Plasmic generates code, the latter focusing on React: https://www.framer.com / https://www.plasmic.app/
But yes it does appear that Penpot for now is the most obvious de facto Figma alternative.
The silver lining of this whole debacle has been that it's brought to surface a lot of other alternatives that folks would not have otherwise considered or looked into, which are actually quite decent I'm finding out.
Just yesterday, while developing a Cordova hybrid app intended to target iOS, Android & Windows Phone 8, this very notion became unsettlingly apparent for me. BTW, http://icons8.com is a great place to do icon recon.
Personally, I like the iOS 6 icon over the iOS 7 version. They're almost the same, except the new one places too much emphasis on "up." For example, when using the Meme Producer app and you want to save the picture to your photo library, the app uses the iOS 7 "action" icon but it feels awkward to then immediately go to the "download" icon to actually save. http://i.imgur.com/YtVd5WZ.jpg
Try lunacy[1] it supports .sketch files and is quite good for the early stage it is in, sometimes it has strange behavior (e.g elements disappearing from the screen after dragging)and the navigation needs a bit of polish etc. hopefully they will get these issues ironed out soon in future updates. There are also online solutions like Figma[2] and Gravit Designer[3] which also has an electron based app for offline use. Not exactly a Sketch replacement but they will get the job done.
1. https://icons8.com/lunacy
2. https://www.figma.com/
3. https://gravit.io/
I have no idea what your priorities are, so it's hard to guess. XD doesn't support Linux either. Apparently https://icons8.com/lunacy runs on Windows and can import and export to Sketch.
Figma of course runs in a browser so works fine on all, and there is a Mobile UI toolkit for it: https://www.figma.com/templates/mobile-ui-kit/
https://www.smashingmagazine.com/2019/04/sketch-figma-adobe-xd-ui-design-applications/
https://www.codeinwp.com/blog/figma-vs-sketch-vs-adobe-xd/
But maybe what you need is... Adobe XD? I personally quite like it.
Unrelated to the primary topic, but I have been trying out a new free (as in beer) app called lunacy [1] for prototyping and the experience has been great.
I am not a UX professional so can't comment on feature parity with Figma etc.
It was however interesting to discover that the native cross platform app was powered by dotnet core & the open source Avalonia framework[2]. It looks quite polished and the framework seems to have come a long way since I tried a few years ago.
[1] https://icons8.com/lunacy
[2] http://avaloniaui.net/