I am sure it's well known that Drupal is a particularly good choice because you do not need to pay for it, quite secure, easy to extend, has a lot of contributed modules and so on. For businesses, there is another advantage: it's a great opportunity to invest, with great returns. Sounds strange? Read on!
I will try to explain here what your company can gain by paying their employees to contribute back to the Drupal project. There are quite a number of ways you can contribute but before indulging into the details, it's worth noting that each contribution adds to some greater goals. In other words, the Drupalsphere grows each time you add something to the project. In plain terms this means that more people know about Drupal meaning more clients and more potential employees to pick from. Measuring this is not easy but Drupal conference attendance in three years grew from 30 people to 900...
<h2>Contributing to the Drupalsphere</h2>
So how can you contribute?
<h3>Work sustainable</h3>
One of the most obvious ways is to release code (as patches). You can fix bugs -- if you found a bug and fixed it for yourself, then releasing the bugfix is usually small work. Also, if you have added features, you can release it back.
<h3>Release new approaches</h3>
The next step is writing your own module and releasing that. If your code is out there then others are going to use it, test it for you and when the next upgrade comes, you will not need to bother with porting your code as it's likely someone else might do it. It should be noted that if you maintain a module, it's a totally accepted practice to link back to your company website from the project page which in itself returns: it might land some client work who looked at the module and needed something based on that and quite obviously they first ask the maintainer.
<h3>Leverage from documentation</h3>
If you write some documentation, others might polish it and next time you hire a developer, she can read that and you saved on her training time. This is definitely an area which always can use a lot of work and you are contributing to the greater good -- very few open source projects are known for stellar documentation, and if we could get Drupal there, it certainly would increase it's market.
<h3>Enhance Drupal's usability</h3>
If you contribute usability wireframes and they are good, you will see others coding it and after the next release you will save a lot of time you build on it. This is, once again a case where you can enhance the positive image of Drupal.
<h3>Help others</h3>
Still another way of contributing is giving support, like answering in the forum. You never know where this might lead -- what if it's your next client who needs to know more before embarking on the project?
<h2>Your Return On Investment (ROI)</h2>
Also, you can have a say where the project -- be it core or a contributed module -- goes. For this, you do not need to contribute code, just reviewing you can have your say. And it's very important. The developers need to imagine how their product is going to be used and if they don't think of a certain use case or they think it's too edge case to be useful, you might find yourself doing something is actually harder with a next release. So definitely go in and give early feedback. That will make it more likely that your work is going to be easier.
However, there is an ultimate return to steadfast contributing which have an immense return in an area not many think of: marketing. Contributing means your brand grows and there is more: Drupal.org has an fairly high Google PageRank rating and if you are mentioned often in blog posts aggregated to Drupal Planet citing your contributions (yes, those blogposts happen), that makes a fantastic boost in your Google results that you could not really earn any other way.