Are there any open source alternative to Landscape?

Better yet, are there any Spacewalk-like software packages that work for both RedHat-based and Debian-based systems?

Solution 1:

In addition to straight Puppet and Spacewalk, there is a puppet-based system called "foreman" that does most of what you're looking for.

while the screenshot does show CentOS and Windows clients, it does have Ubuntu support

I've used it and I recommend giving it a go.

Solution 2:

As of 2019, Landscape on-premises is free for small installations (10 servers + 10 VMs).

Installation instructions can be found here: How do I install Landscape for personal use?

It is also open source, with source code available on launchpad.

Solution 3:

According to the FAQ, Spacewalk supports Debian, so it may support Ubuntu as well.

If you are looking for configuration options for a large environment, you could take a look at Puppet.

Solution 4:

You can try out vFense , which is still in Beta, but it supports Ubuntu, Debian, RedHat, OS X and Windows. vFense is an open source cross platform patch management tool, that is also cross referencing vulnerability data from Ubuntu, Windows, and soon RedHat as well.

Full disclosure, I am the lead developer of the project.