Ah, OpenHAB was the other one I couldn't remember.

I'm guessing you are not in the US. Have never heard of a lot of those and I see most are based in Europe.
If you are in Europe, you should give Loxone a look. It only recently came to the US a few years ago but has been in Europe for awhile. They offer an upscale system.

A note on open source and customizable GUI = Most professional systems are not going to be open source and they are going to be more limited in the customization. What they do offer is 24/7 reliability and solid performance.

Most open source and fully customizable GUI systems are usually more flaky. They are more DIY and require more attention to keep them up and running. This is great for people who like to play with things. Not great for people who just want an automation system installed and not have to think about it (not something a professional would offer a customer).