We did not previously use a different solution. 

Prior to my arrival, the company was using Ghost and Windows Deployment Services, WDS, however, WDS was subsumed into System Center and Ghost just no longer really served our needs.

I have almost 18 years of monitoring and Systems Management experience, so I have used a variety of products. For the Wintel estate currently SCOM is the best fit for the interim in terms of cost, feature, and function. 

We were using an open-source solution, but I can't remember the name.

We did use a different solution, however, it was so long ago, I don't recall the name. It's been ten years. The switch of technology made us originally switch solutions.

We used WhatsUp Gold, but System Center came with our Microsoft license.