So far I have come up with the following:
There could be other companies as well in non-software fields.
For a better explanation of the context of this question, see http://www.brightjourney.com/q/thinking-startups-building-blocks-larger-corporation
There are a series of game related companies, that build products for other games companies, such as (a small taste, plenty more then listed below)
There are also hosting platforms for code like
I guess it comes down to what exactly you mean by platform. The phrase is too general, and I've been limiting my answer to simply software platforms (let alone say electronics hardware development platforms, or manufacturing platforms, or trading platforms).
There are plenty of examples in pretty much any business or academic focused area that has vendors, both startup and established, providing significant ground work for other groups to achieve their goals. In addition, their profitability and growth is wildly variable consider the scope of groups that "platform" can cover.
Edit: One more, you are most likely aware of Salesforce. Even if they don't like to admit it, they are a software platform.
Seems to me anyone with a good API and plugin/app ecosystem would qualify - Facebook, Twitter, WordPress, Atlassian, etc.
And like Scott mentioned, Salesforce is a huge platform with their Force.com offering. I haven't really followed the company, but that seems like a big upside for them. A lot of developers targeting the business market are building on their platform.