In many ways, All of Them.
- Microsoft. Bought 86-DOS, turned into MS-DOS (which Gates did not invent), which in turn was an improved clone of CP/M. Did not invent BASIC, either. Office in many ways built in part on top of Lotus 1–2–3’s success. Windows after/alongside Mac OS. Etc. etc. etc.
- Facebook. MySpace, Friendster, whatever.
- Google. Yahoo, Altavista, whatever.
- Salesforce. Maybe not a fast follower, but already Siebel Sales.com, etc.
- Apple. Copied Xerox, Altair, everything.
- Square. Very innovative, but a mobile PayPal, etc.
- Marketo, Hubspot, More. Neither invented the space.
- Slack. Very innovative, especially in integrations. But we had so many point solutions before, and since.
This may not be 100% fair and certainly is an over-simplification. But software is about innovating by copying what works, but doing it a lot better, and taking advantage of paradigm changes (Cloud, Mobile, etc.).
This is why you can start so young.
And even if you really do something brand new, you usually still copy UI/UX elements and paradigms that are already well proven.
