I’ve used HP’s StorageWorks DAS arrays quite a bit in my years as a data center engineer. They’re perfect for small implementations and very scalable.
I’ve also seen Dell’s PowerVault products but not used them much.
For big iron storage, though, EMC seems to have a lock on marketing. I know they undersold IBM by an extreme margin at my last employer just to get their foot in the door, something like 90%, and with the amount of money they have, they throw around their weight quite freely.
Like I said, I acknowledge other competitors, but EMC is a pretty tough competitor to try to squash or avoid.
Which vendor is typically determined by the need of the overall project or application, and although EMC is a favorite for some of the larger, it is by no means the only solution. HP’s aren't one of my favorites, but they are usable. LSI made very powerful SAN’s that have been discontinued by NetApp after they acquired them (CTS2600 was a monster), and that's a shame. IBM rebranded that unit from LSI and actually forced them to “throttle down” the performance as it was outperforming their flagship DS8300. Other vendors have also resold that LSI unit and although NetApp has to continue to produce them for now, eventually that tech will fade away as NetApp decided they wanted to sell their overpriced hardware at a much better margin.
EMC is the primary player and will likely continue to be so for many years to come, but new tech is entering the picture from other vendors and they have a lot of work to do to keep themselves on top of the pile.