What are some alternatives?
When comparing Heroku and OpenNebula, you can also consider the following products
Microsoft Azure - Windows Azure and SQL Azure enable you to build, host and scale applications in Microsoft datacenters.
Proxmox Virtual Environment - Proxmox VE is an open-source server virtualization platform to manage KVM virtual machines, LXC, software-defined storage and networking, HA clustering, and multiple other out-of-the-box tools on a single solution.
DigitalOcean - Simplifying cloud hosting. Deploy an SSD cloud server in 55 seconds.
VMmanager - VMmanager is a QEMU/KVM server virtualization management software, which presents perfect tools for creating virtual machines, providing VPS services, and building cloud infrastructure.
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
OpenStack - OpenStack software controls large pools of compute, storage, and networking resources throughout a datacenter, managed through a dashboard or via the OpenStack API.