The first continuous integration (CI) system I saw in use was back in the mid 2000s. Back then I don't think it was even called "continuous integration". It was just a way to test that builds worked, run unit tests and the send alerts if things had broken. For long builds that would usually be run overnight. It also wasn't anything formal, just some arbitrary and completely bespoke scripts on a server.
More recently I was discussing whether it would be a good idea to have continuous integration for the Freedombone project. Since making images involves building a minimal Debian and then installing and configuring other apps this can take quite a while. A couple of hours on a fairly powerful machine. At first I thought there wouldn't be much to be gained by doing this, but on second thoughts maybe it might be useful if I can find or requisition an old desktop machine to do it. At present testing of Freedombone builds is very manual and quite laborious.
I know that Lars Wirzenius has been developing a CI system called ick. As a side note you might also want to check out his Getting Things Done for Hackers. Ick looks ok and I could probably use it, but the code looks rather complicated and I'm not a huge fan of yarns as a software validation methodology, although I can understand why developers might be attracted down the path of making things as similar to natural language as possible. I was thinking that for my purposes CI doesn't need to be this complicated, since there's only a small set of things which it needs to do.
So I spent roughly a day writing a CI system and the result is BirbCI. This is really a single command implemented as a bash script which can set up systemd daemons to run builds and then report the results as a html page. It's about the simplest possible CI system at roughly 330 lines of script including comments. I also blinged it up a bit with a background image and coloured icons and text representing build status, so that it doesn't look too sparse and utilitarian.
This should be good enough to start thinking about making some sort of Freedombone build server. It's not a super-high priority, but maybe something to work on as a background task in the next few weeks or months.