In the US, there are three big grocery POS companies. IBM, NCR, and Retalix (update: NCR has since purchased Retalix). They all have strengths: IBM's 4690 platform uses a purpose-built operating system that helps with stability and avoids some security problems. Retalix is hardware agnostic and uses modern APIs and a .NET backend. NCR is generally considered to have the most powerful loyalty platform.
With 55,000 different SKUs in the typical grocery store, complex laws around which products are taxed, which can be discounted, manufacturer's coupons, loyalty card programs, supply chain management, cash management, and the necessity of running 24 hours a day, 7 days a week…grocery POS tends to be much more complex than any other retail vertical. And, unfortunately, probably too complex for Square on an iPad, Rex Dixon. 🙂
There are a number of less-significant players; these are companies that may have begun in other retail verticals and are gradually adding features required by grocery (loyalty, manufacturers coupons, variable tax rates, HIPAA compliance on pharmacy, limitations on alcohol or tobacco, etc.). Baris Alyanak mentions Microsoft's Retail Management system, while not installed at any Top 100 grocery chains, does have a presence with some niche grocery retailers.