Adobe XD is a free software for making User Interfaces, this is quite good for developers who don’t want to spend much on their tools, but make quality designs.
Adobe Illustrator is an extension of making symbols and logos etc for Adobe XD, but this is more advanced and has a subscription payment.
Affinity Designer is what you’ll go for if you’re a die hard UI designer, it has the tools, features and is much easier to use than Adobe XD, though this what you call “You get what you pay for”. I believe in most workplaces, they use this application? a friend told me, so yeah.
There are probably more programs out there(Photoshop, Indesign, Sketch etc.) , but these are the common-ish ones… I also made this guide on how to create nice UI designs, so you might wanna check this out:
For the best result in your UI design projects, I’d suggest using a vector illustration software. These programs use mathematic equations and geometrics to create really clean, scalable images - perfect for UI!
The best free option out there is probably Inkscape. It’s a really handy vector illustration software and you can accomplish quite a lot in it. I actually used it to create the UI in some of my earlier projects. There’s a bit of a learning curve and it takes a while to get used to but once you get the hang of it, it’s really versatile. If you’d like to learn the basic, you can find a good tutorial playlist here!
If you feel like making a once-off purchase for a more professional software with a few extra features and a cleaner interface, I’d highly recommend Affinity Designer. It’s been my vector illustration software of choice for the past few years. As @wevetments has already touched on, its used in many workplaces and is an industry-standard tool. It doesn’t have quite as much functionality as Adobe Illustrator (text manipulation is particularly difficult) but for the most part, you can create whatever you want in it.
I know there is gimp or Inkscape as free software. Gimp is more for photo editing as for Inkscape it is a vector software which is more what you are looking for.
What I personally I use for designs though is Affinity Designer. It is an amazing software especially for mac, I believe it is compatible on pc as well. However it isn’t free, though I think you can get a free trial.
Like the others, I also use Affinity Designer, the one time payment is really nice. And it has a lot of the tools you’ll usually find in Illustrator or InDesign. However, there are some tools that are missing from the workflow and when you find them it sucks really. That being said it can open PSD, You can export as a PSD as well.
Unfortunately it’s not really good at making desktop icons… (I know, what a bummer).
On top of that I use Figma. Figma is good for mock up, and concepts. And you can even use FontAwesome inside of Figma. You can set up simple logic to make interactive website designs, or interactive UI designs. And then you can show it as a presentation.
There’s comments that you and anyone in your team can place anywhere. It’s free I believe for up to 2 team members.