NixOS friendly hosters

1st class NixOS support
GandiCloud VPS	Start a NixOS server with a single command or by few clicks in the web UI: it is a provided and supported server image.
VPS2day	When ordering or configuring, the "Manual Install (Advanced)" option includes NixOS. Installation is done through a web-based console.
vpsFree.cz	A non-profit association providing free as in freedom VPS services for its members. NixOS is available in the registration form and administration interface. See NixOS at kb.vpsfree.org for more information. See nixops example on howto setup a server using nix 2.7 and nixops 2.0.
Rackzar.com	South African Hosting Provider offering NixOS on all VPS plans as a drop down OS during checkout.
Support NixOS via custom ISO
BuyVM	BuyVM no longer provides first party support for NixOS. You can install NixOS using the custom ISO functionality, however the resulting install will need some form of static IP configuration.
Hetzner Online	From Hetzner's rescue image one can boot into the nixos installer using a custom kexec image. The installation can be fully automated using nixos-anywhere. Hetzner also provides an interface to upload your own ISO-images. Also here you may want to build your own iso-image, which has openssh with ssh keys due the lack of a remote console. An easier method to install NixOS on Hetzner, is to use the existing integration into NixOps. Also see Install NixOS on Hetzner Online for future information
Hetzner Cloud	Hetzner Cloud has NixOS ISO images that can be mounted on a running instance. When creating the VM, you have to choose a different initial distribution and than select the NixOS under "ISO images" in the VM instance page to reboot into the NixOS installer. An alternative approach is using nixos-anywhere than can also use other Linux distributions as a base and convert them into NixOS installation. See also Install NixOS on Hetzner Cloud.
LiteServer	NixOS ISO is available from the control panel. Also see Install NixOS on Liteserver.
BandwagonHost	NixOS ISO is available from the control panel.
netcup	NixOS ISO can be uploaded via FTP to be available in the server control panel. (in the SCP, navigate to: 'Media' → 'Images') See the official netcup documentation. Step-by-step installation instruction can be found here: NixOS on Netcup (In English). It may be necessary to create a bios-boot partition for grub.
Online	A NixOS ISO can be use with iDrac (Dell) or iLo (HP). You can upload your own iso or use virtualmedia. You must have a Dell or HP server. Here is the documentation if you don't know DRAC at Online.
Inception Hosting	Inception hosting no longer provides first party support for NixOS. You can install NixOS using the custom ISO functionality
Linode	no longer provides first party support for NixOS. You can install NixOS using the custom ISO functionality. Install and Configure NixOS on a Linode
RamNode	RamNode no longer provides first party support for NixOS. You can install NixOS using the custom ISO functionality.
Vultr	Vultr no longer provides first party support for NixOS. You can install NixOS using the custom ISO functionality.
PHP-Friends	NixOS ISO is available from the control panel.
Cloud.co.za	NixOS ISO is available from a drop-down menu option when selecting ISO image to mount a image for the CD-ROM device. You may also request their helpful support team to mount a specific ISO image. Using the graphical installation works best as you will need to set the static ip address, gateway and nameserver as provided to gain internet access. During the installation setup and you will configure your login to be password-less in order to access the server via the VNC console. On the first boot, to complete the configuration for the networking (hostname, static ip, gateway, nameservers, etc), the ssh access for your user and lastly remove the password-less login.
Other means of installation
Amazon EC2	See NixOS on Amazon EC2
DigitalOcean	nixos-infect can be used to transform an existing installation into NixOS. This method will be also used in NixOps.
Linode	Linode Guide.
Genesis Cloud	After unmounting /boot, nixos-infect can be used to transform an existing Ubuntu 16.04 installation into NixOS. This process can be fully automated (example) when using the API by supplying the unmount and nixos-infect commands in a custom startup_script.
Google Cloud Platform	See Installing NixOS on GCE
Online	See Install NixOS on Online.Net
OVH / Kimsufi / So you Start	Blog posts for OVH and SoYouStart
Rackspace	See Install NixOS on Rackspace Cloud Servers
Scaleway	The kexec method (see Install NixOS on Scaleway X86 Virtual Cloud Server) works well, but bear in mind it only works with the 'virtual' class of servers, as they run under a hypervisor which attaches the disks before the kernel boots. The 'bare-metal' servers rely on a special Linux kernel booting to attach network /dev/nbdX drives which works only with Scaleway supplied images. There is a image proposal to add full NixOS support.

Hoster-agnostic means of installation

This section links to or explains methods that can be used on various providers. As always, with unsupported hosts and unsupported distributions, your mileage may vary.

Not all providers allow to upload custom images or provide NixOS images for installation. However there are a few ways to install NixOS anyhow. nixos-in-place and nixos-infect are generic approaches to install NixOS from an existing Linux. Another approach is to use a kexec-based image as is done with nixos-anywhere. This also allows for changing the underlying filesystem. See Install NixOS on a Server With a Different Filesystem.

See also the NixOps project, which also provides interfaces to different cloud providers.