The thing that these challengers have in common is that they're released as open source software, meaning that developers all over the world have helped develop them.
For instance, Cassandra started at Facebook, but eventually made its way to open source, where the Apache Software Foundation took over its development. Hadoop, too, traces its roots back to Yahoo. Docker, the hot new open source software container technology, was borne from a now-defunct cloud computing service provider called dotCloud.
For customers with the technical chops to effectively put it to work and customize it to their specific needs, free software is a great deal, compared to the many hundreds of thousands of dollars that vendors like Oracle, EMC, or Microsoft can charge for similar products.
And since so many of these technologies started at the big web companies, they're already vetted and made to scale up and up. And with the help of developers all over the world contributing their own concepts and code, this software only gets better.
"Over time, the overall quality of open source software is generally high as more people test and use it," says Sri Ramanathan, the CTO of Kony, a company that helps enterprises build custom applications.
Plus, open source software generally runs on cheaper hardware, versus the expensive hardware that required to run a big Oracle database.
Of course, the Oracles and IBMs of the world do have one advantage over free and open source software: they offer their customers the security blanket of knowing that if something goes wrong, there's a phone number you can call to get help. That's not the case with free software.
This is why a lot of startups have found success by taking free technology and turning it into a commercial product. They offer business support, everything from help installing it to help with troubleshooting it, and, sometimes, extra features as well. And they can still charge a lot less than the bigger vendors for their software.
DataStax, for example, takes the Cassandra database software and makes it more consumable to enterprises. In so doing, the company was able to charge a customer $90,000, versus the $500,000 that Oracle was charging that customer for a similar project, the company recently claimed to Bloomberg Business.
Similarly, MongoDB, which offers premium support services for its namesake open source database technology, claims that it can save a customer 70% off their Oracle costs.
In fact, that same Bloomberg Business report claims that of the 20 startups worth $1 billion or more that it surveyed, none of them were using Oracle databases in a major way.
"Open source is actually a powerful strategy and not something to be afraid of," says Docker SVP of Product Scott Johnston.
A final point: As recently posited by Redpoint investor Tomasz Tunguz, open source-based startups may have an easier time selling straight to the developers at prospective customers - since anybody can download and look at the source code of the project, these developers understand exactly what it is that they're buying, and can even try it out for free if they want.