Any that are available thru distribution. I would make a list but 1st we would need to define 'control system'. That will vary a lot depending on the person and their perception.

Technically Harmony is a control system. But many here would argue against that, even if it is true.
Same with URC distribution lines, they are control systems but because they lack certain control protocols (or ability to write your own drivers), some don't consider them as such - even though they are.
RTI.
Crestron has minimums for factory direct dealers. But as I understand it, you can get equipment from places without being a dealer and places you can access the software if you really look around. Plus they don't lock the dealer (any person) out of the system - as long as you have the correct credentials.

Really, if you have access to the software and have the correct credentials (if applied), ANY of the popular control systems can be worked on.
C4 is the ONLY control company (that I'm aware of) that can and will lock a person (dealer, programmer, etc.) out, not allowing them to access the system at all. They own it!
Doesn't matter if the customer payed for it, if the dealer payed for it, or anyone that has actual possession of the system; C4 has ultimate control of that system and can choose who to allow access to it at anytime.

Which IMO brings up the whole legal issue of intellectual property. If someone pays for a product 'in full', how the f^ can the courts be allowing these dirty companies to essential still own the hardware still (thru software ransom). I mean, isn't this just legalized ransomware! Shouldn't it be illegal!
To be clear: locking the hardware so it is not accessible. Intellectual property applies to the programming on the hardware. A company should have the right to not allow access to 'their' programming if they so choose. Customer or another programmer would have to start from scratch. But they could use the hardware.