Tested on Devuan Jessie alpha2 64-bit.

For the 3D (OpenGL) acceleration features of the ATI open source drivers it is required to install a non-free firmware package.

1. Edit /etc/apt/sources.list by adding the suffix 'contrib non-free' next to 'deb <URL> jessie main'. Example:

   deb http://nl.mirror.devuan.org/merged/ jessie main contrib non-free

2. Update the list of packages:

   apt-get update

3. Install the non-free firmware package:

    apt-get install firmware-linux-nonfree

4. Restart the system.
5. To check for success either start a 3D program/game or install the mesa-utils and test with glxinfo:

   apt-get install mesa-utils
   glxinfo | grep OpenGL

6. The output should show an “OpenGL renderer string” mentioning the installed ATI graphics adapter and should not contain 'llvmpipe'. Example:

   OpenGL renderer string: Gallium 0.4 on AMD RV730

Sources:
Debian - Wiki - AtiHowTo
Debian - Wiki - Mesa

Tested on Devuan Jessie alpha2 64-bit.

The current (as of 2015-06-30) version of Steam needs some manual configuration to get it to work on Devuan.

1. Add MultiArch support for i386 architecture (on x86_64) and update the package list:

   dpkg --add-architecture i386
   apt-get update

2. Install prerequisites:

   apt-get install libgl1-mesa-dri:i386 libgl1-mesa-glx:i386 libc6:i386 curl zenity zenity-common libcurl3 libwebkitgtk-3.0-0

3. Download steam_latest.deb from http://store.steampowered.com/about/.
4. Install steam_latest.deb:

   dpkg -i steam_latest.deb

5. Starting Steam now will only show:

   Setting up Steam content in /home/bas/.local/share/Steam
   Running Steam on devuan 1 64-bit
   STEAM_RUNTIME is enabled automatically
   Installing breakpad exception handler for appid(steam)/version(0)
   libGL error: unable to load driver: swrast_dri.so
   libGL error: failed to load driver: swrast

   At which point you should kill it with Ctrl+C.

6. This is caused by the libstdc++ library shipped with Steam not being compatible with Devuan. To fix:

   cd ~/.local/share/Steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu
   mv libstdc++.so.6.0.18 libstdc++.so.6.0.18.remove.by.user
   rm libstdc++.so.6

7. Now start steam and allow it do download it's updates. When Steam is again at the 'libGL error: failed to load driver: swrast' point kill (Ctrl+C) it and again rename the libstdc++ files:

   cd ~/.local/share/Steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu
   mv libstdc++.so.6.0.18 libstdc++.so.6.0.18.remove.by.user
   rm libstdc++.so.6
   cd ~/.local/share/Steam/ubuntu12_32/steam-runtime/amd64/usr/lib/x86_64-linux-gnu
   mv libstdc++.so.6.0.18 libstdc++.so.6.0.18.remove.by.user
   rm libstdc++.so.6

8. Start steam. It should now work and allow you to login.

Sources:
noflex.org - Fix steam error libGL error: failed to load driver: swrast in debian
LinuxConfig.org - Installation of Steam Client on Debian Jessie 8 Linux 64-bit
Debian - Wiki - Steam

1. Stop Slim:

   /etc/init.d/slim stop

2. Login as root.
3. Start the driver installation:

   sh NVIDIA-Linux-x86_64-358.16.run

4. Go through the wizard for driver installation. Don't forget to register the driver with DKMS.