They're all protected by patents. All of them. I can pretty much guarantee you that any of IBM, Microsoft, Google, Oracle, Intellectual Ventures, and many others have sufficient patent coverage to be able to sue you for anything you could possibly build that involves software in any way.
Lawsuits aren't always about winning. Sometimes they're about bullying and showing dominance. Sometimes they're about stalling a competing product while you finish your own. In the case of a big company or troll going after a small company, drawn out legal battles are used to bleed the company dry until they settle or go bankrupt.
[edited to scope it to software]