In texmf-dist/doc/latex/arabi/user_guide.pdf, pages 50–51 give samples of fonts from Microsoft and from arabeyes.org (there are .ttf versions of the latter in Debian’s fonts-arabeyes package). And the documentation for dad shows examples of its output. Of course, with xetex and luatex, you can use any unicode, OpenType font, whether free or commercial. Here are some free fonts:
\documentclass{article}
\usepackage[novoc]{arabluatex}
\linespread{1.25}
\newcommand{\sample}{\arb{الأفكار الخضراء عديمة اللون تنام بغضب}}% http://www.omniglot.com/language/phrases/colorlessgreenideas.htm
\begin{document}
Amiri: \sample% arabluatex defaults to Amiri
\newfontfamily\arabicfont[Script=Arabic]{Aref Ruqaa}
Aref Ruqaa: \sample% https://github.com/khaledhosny/aref-ruqaa
\newfontfamily\arabicfont[Script=Arabic]{Cairo}
Cairo: \sample% https://github.com/Gue3bara/Cairo
\newfontfamily\arabicfont[Script=Arabic]{Hussaini Nastaleeq}
Hussaini Nastaleeq: \sample% https://github.com/khaledhosny/hussaini-nastaleeq
\newfontfamily\arabicfont[Script=Arabic]{Jomhuria}
Jomhuria: \sample% https://github.com/khaledhosny/Jomhuria
\newfontfamily\arabicfont[Script=Arabic]{Lateef}
Lateef: \sample% http://software.sil.org/arabicfonts/
\newfontfamily\arabicfont[Script=Arabic]{Mada}
Mada: \sample% https://github.com/khaledhosny/mada
\newfontfamily\arabicfont[Script=Arabic]{Noto Kufi Arabic}
Noto Kufi: \sample% https://www.google.com/get/noto/
\newfontfamily\arabicfont[Script=Arabic]{Noto Naskh Arabic}
Noto Naskh: \sample% https://www.google.com/get/noto/
\newfontfamily\arabicfont[Script=Arabic]{Reem Kufi}
Reem Kufi: \sample% https://github.com/khaledhosny/reem-kufi
\newfontfamily\arabicfont[Script=Arabic]{Scheherazade}
Scheherazade: \sample% http://software.sil.org/arabicfonts/
\end{document}

You can get a list of Arabic fonts available on your computer by typing fc-list :lang=ar on the command line, and you can search for all Arabic Google fonts.
Update: Here’s the brand new FiraGO, an extension of Fira Sans which adds support for Arabic, Devanagari, Georgian, Hebrew, and Thai to the already impressive range of Fira Sans:
\documentclass{article}
\usepackage[novoc]{arabluatex}
\newcommand{\sample}{\txarb{الأفكار الخضراء عديمة اللون تنام بغضب}}
\begin{document}
\newfontfamily\arabicfont[Script=Arabic]{FiraGO}
FiraGO: \sample% https://github.com/bBoxType/FiraGO
\end{document}

Update two: Here is the new Kafa Black:

and its stylistic alternate:

Update three: Recently, IBM Plex Arabic has joined the IBM Plex family:

Update four: Here’s Aref Ruqaa compiled with lualatex-dev:

Update five: Here are Cairo, El-Messiri, and Lemonada, by Mohamed Gaber:



Update six: Abd ElRady, by AhmED ElqSas:

Update seven: As of 5 November 2020, there’s also Scheherazade New, which can be installed alongside the older Scheherazade; the new version has resized glyphs:
