NixOS friendly hosters

1st class NixOS support

packet.net	NixOS on Packet
linode	Install and Configure NixOS on a Linode
vps2day.com	you have to click on "Manual Install (Advanced)" to get list of all OSes available to install including NixOS

Support NixOS via custom ISO

Vultr	nixos.wiki is running on a NixOS VPS at Vultr
Netcup	NixOS ISO can be uploaded via FTP to be available in the server control panel. (in the SCP, navigate to: 'Media' → 'Images')
Hetzner	NixOS ISO can be provided by the support. Just request them to make the ISO available via the support formular (provide a download link to the ISO)
LiteServer	NixOS ISO is available from the control panel.

Other means of installation

AWS EC2	See NixOS on Amazon EC2
online.net	See Install NixOS on Online.Net
Rackspace	See Install NixOS on Rackspace Cloud Servers
OVH / Kimsufi / SoYouStart	See this blogpost by Augustin Borsu
Google Cloud Platform	See Installing NixOS on GCE

Hoster-agnostic means of installation

The Installing on VPS & Cloud Providers section of the installation guide page explains methods that can be used on various providers. As always, with unsupported hosts and unsupported distributions, your mileage may vary.