Charles River Development
The v9 release seems to be gathering some momentum. Buy-sides are being offered a "SaaS" model of sorts but this looks more like hosting than a true SaaS model. Always rumours about potential sale. Most recently Thomson Reuters were linked.
The buy-side offering is a solid performer but has not been picking up sizeable new business recently.
Coverage: Equities (single stock, DMA, Algo, PT), F&O (limited)
[Update 13/05/2016 - spoke with the CTO/CEO - it's all SaaS and a real open source culture within the firm. When I get time I will add more detail on this firm.]
Coverage: Equities (single stock, DMA, Algo, PT), F&O